Stress Resilience: Molecular and Behavioral Aspects 0128139838, 9780128139837

Table of contents :
Cover
Stress Resilience
Copyright
Dedication
Contributors
About the editor
Preface
Acknowledgments
1 -
A life-course, epigenetic perspective on resilience in brain and body
Introduction
What is stress?
Definition of stress, allostasis, and allostatic load
Protection and damage as the two sides of the response to experiences
Brain as the central organ of allostasis and allostatic load/overload
Plasticity and vulnerability of the hippocampus
Cellular processes involved in structural plasticity
Extension of stress effects to amygdala and prefrontal cortex
Other mediators of structural plasticity
Glucocorticoids as key players in PTSD vulnerability
Sex differences
Lessons of an ever-changing brain from gene expression
Epigenetics: two meanings that are both important for prevention and treatment
Individual differences and experiences throughout the life course
Early-life experiences
Intervention
References
2 -
Cognitive and behavioral components of resilience to stress
Resilience: one of many possible responses to stress or trauma
Cognitive and behavioral components of the psychosocial factors associated with resilience
Optimism
Cognitive flexibility
Active coping skills and a strong social support network
Physical activity
A personal moral compass
Cultivating psychosocial factors to promote resilience
Encourage optimism, attend to pessimism, and aspire for flexibility
Face your fears
Connect with a resilient role model
Form and maintain a supportive social network
Attend to physical health and well-being
Attend to your personal moral compass; identify and foster your character strengths
References
3 -
Resilience as a process instead of a trait
Introduction
Learning-to-cope training
Learning to cope inferred from hormones and behavior
Neurobiology of learning to cope
Limitations
Conclusions
References
4.-
The brain mineralocorticoid receptor: a resilience factor for psychopathology?
The brain mineralocorticoid receptor
Mineralocorticoid receptor activation and neuronal activity
Mineralocorticoid receptors and cognitive function in rodents
Pharmacology, genetic variation, and vulnerability to psychopathology in humans
The mineralocorticoid and hypothalamus-pituitary-adrenal axis activity
The mineralocorticoid, learning, and stress appraisal in humans
The mineralocorticoid receptor and resilience and vulnerability for psychiatric disorders
Concluding remarks
Brain mineralocorticoids important for resilience?
Future directions
References
5.-
GABAB receptors, depression, and stress resilience: a tale of two isoforms
Introduction
The impact of stress-related psychiatric disorders and their treatments on GABAB receptor density, gene expression and function
Effects of antidepressants on GABAB receptor density in rodents
Effects of antidepressants on GABAB receptor function in rodents
Clinical evidence of altered GABAB receptor density and function in depression and the antidepressant response
Alterations in GABAB receptor density and function in animal models of stress and depression
Effects of GABAB receptor modulation on depression-like behaviors
The role of GABAB1 receptor subunit isoforms in stress resilience
Potential mechanisms underlying the differential roles of GABAB1a and GABAB1b receptor subunit isoforms in stress resilience
The serotonin neurotransmitter system
The hypothalamic-pituitary-adrenal axis
Location, location, location…
Adult hippocampal neurogenesis: a mechanism for resilience?
Conclusions
Acknowledgements
References
6 -
Sex differences in the programming of stress resilience
Introduction
Sex x life span interaction in producing resilience
Sex hormone x life span interaction in producing resilience
Sex chromosome x life span interaction in producing resilience
Conclusion
Acknowledgments
References
7 -
Active resilience in response to traumatic stress
Resilience—a passive lack of effect or an active response?
Two isozymes of glutamic acid decarboxylase
GAD genes are regulated in response to fear and stress
GAD is required for resilience
GAD65 haplodeficiency conveys stress resilience
GAD65 and stress resilience—a complex picture
Summary
Acknowledgments
References
8 -
Rhythms of stress resilience
Hypothalamic-pituitary-adrenal axis rhythms
Circadian rhythm and stress response
The importance of pulsatility for hormonal and behavioral response to stress
Glucocorticoid rhythms and the response to stress in physiological and pathological conditions
Cortisol rhythms and stress resilience in humans
References
9 -
Mitochondrial function and stress resilience
Introduction
The mitochondrion
Mitochondria in neurotransmission and synaptic plasticity
Mitochondria and glucocorticoids
Mitochondrial dysfunction in stress-related disorders: human studies
Stress effects in mitochondrial function: animal studies
Promoting stress resilience through activation of mitochondrial function
Conclusions and future perspectives
References
10 -
Understanding resilience: biological approaches in at-risk populations
Introduction
Definitions and measurement of resilience
Biological facets of resilience
Genetics
Candidate studies
Genome-wide unbiased studies
Physiology
Neuroimaging
Resilience as a multidimensional trait
Conclusion/summary
Acknowledgments
References
11 -
Stress resilience as a consequence of early-life adversity
Introduction
Early-life stress—definition of the term
Early-life stress is a risk factor for psychiatric disorders
Early-life stress shapes adult phenotypes
What is the rationale for shaping adult phenotypes by early-life experiences?
Evidence for the match/mismatch theory in humans
Evidence for the match/mismatch theory in animal studies
Conclusions
References
12 -
Mechanisms by which early-life experiences promote enduring stress resilience or vulnerability
Introduction
The degree of predictability of maternal care influences long-lasting cognitive and emotional resilience or vulnerability
Studying early-life experiences experimentally
Disrupted maternal care
Augmented/predictable maternal care
Cognitive and emotional outcomes of early-life experiences
A spectrum of cognitive consequences of early-life experiences
Emotional consequences of early-life experience
Mechanisms by which early-life experiences elicit enduring changes in neuronal, circuit, and behavioral functions
Stress-sensitive neurons in the hypothalamus are influenced by early-life stress as well as by augmented early-life experience
Memory consequences of early-life stress and experiences—a hippocampal story
Early-life experiences affect a number of brain systems
How the consequences of early-life experience are encoded long-term: transcriptional and epigenetic mechanisms
Conclusions
Acknowledgments
References
13 -
Child abuse and neglect: stress responsivity and resilience
Stress responsivity physiology
Hypothalamic-pituitary-adrenal axis physiology
Childhood maltreatment influence on hypothalamic-pituitary-adrenal/sympathetic nervous system response to stress
Sympathetic nervous system
Glucocorticoid feedback regulation of stress responsivity
Epigenetics of stress responsivity
Stress responsivity neural circuits
Stress responsivity and inflammation
Stress responsivity and resilience
Resilient stress responses: CRFR1/OPRL1/5HTLPR/BDNF/NPY/DHEA
Treatment/implications/future
Financial Disclosures
References
14 -
How genes and environment interact to shape risk and resilience to stress-related psychiatric disorders
Introduction
Prenatal development
Infancy
Childhood
Adolescence
Adulthood
Conclusions
References
15 -
Molecular characterization of the resilient brain: transcriptional and epigenetic mechanisms
Introduction
DNA methylation
Chromatin modifications
MicroRNAs
Transcription factors
Immune-related processes
Neurotrophic factors
Circuit-related molecules
Genome-wide studies
Future directions
Summary
References
16 -
The role of the CRF-urocortin system in stress resilience
Introduction to the corticotropin-releasing factor/urocortin system
The corticotropin-releasing factor/urocortin system as a critical mediator of the behavioral stress response
The corticotropin-releasing factor/urocortin system mediates stress vulnerability caused by chronic stress exposure
Corticotropin-releasing factor/urocortin system mechanisms influencing resilience
Corticotropin-releasing factor system genetic variance x environment interactions
Epigenetic regulation of corticotropin-releasing factor system expression
Stress regulation of CRFR1 availability
Stress-induced changes in CRFR2 expression
Corticotropin-releasing protein–binding protein function
Alterations in intracellularly activated signaling pathways
Conclusion
References
17 -
Intergenerational transmission of stress vulnerability and resilience
Introduction
Foundational populations: studies of the Dutch hunger winter and holocaust survivor offspring
Maternal versus paternal transmission
Maternal transmission
Paternal transmission
Hypothesized mechanisms of transmission
Intergenerational transmission of resilience
Conclusions and future directions
References
18-
Stress and its effects across generations
What is epigenetic inheritance?
Why epigenetic inheritance?
Germline versus non–germline transmission
Germline-dependent transmission
Non–germline transmission
Preclinical and clinical studies of inheritance of stress susceptibility
Inherited effects of stress in rodents
Stress in utero
Early life stress
Adolescent and adult stress models
Environmental enrichment
Inter- and transgenerational stress effects in humans
In utero
Postnatal stress
Other environmental factors that may impact stress response across generations
Drugs of abuse
Relevance of studying inheritance of the effects of stress for society
References
19 -
Corticolimbic stress regulatory circuits, hypothalamo–pituitary–adrenocortical adaptation, and resilience
Glucocorticoid signaling, stress, and reslience
Neural circuitry of stress regulation
Limbic regulation of hypothalamo–pituitary–adrenocortical axis stress responses: hippocampus, amygdala, and prefrontal cortex
General organizational scheme of limbic stress regulation
Hippocampus
Amygdala
Medial prefrontal cortex
Integration of hippocampal, prefrontal, and amygdala projections
Bed nucleus of the stria terminalis
Paraventricular thalamus
Hypothalamic and brain stem circuitry
Toward a neurocircuitry of stress resilience
Acknowledgments
References
20 -
Biomarkers of resilience and susceptibility in rodent models of stress
Introduction
Experimental strategies
Prospective strategies
Retrospective strategies
Potential additional biomarkers
Conclusion
Acknowledgments
References
21 -
Maladaptive learning and the amygdala—prefrontal circuit
Modeling stress and anxiety disorders through behavioral paradigms of learning
Cognitive and physiological components of emotional learning
Associative learning in the amygdala: a preference for aversion
Associative learning in the medial prefrontal cortex: mixed selectivity encoding
The prelimbic and infralimbic subregions of the medial prefrontal cortex in associative learning
Overview of amygdala—prefrontal communication during aversive emotional learning
Directionality of amygdala–prefrontal communication during acquisition of stimulus discrimination
Amygdala–prefrontal communication during recall of learned associations
A unified view of mPFC–BLA circuit function in adaptive learning
References
Suggested Reading
22 -
Endocannabinoid signaling and stress resilience
Impact of stress on endocannabinoid signaling
Endocannabinoid regulation of the stress response
Endocannabinoid signaling in the context of susceptibility and resilience to repeated stress
Conclusions
References
Index
A
B
C
D
E
F
G
H
I
L
M
N
O
P
R
S
T
U
V
W
Back Cover

Citation preview

STRESS RESILIENCE MOLECULAR AND BEHAVIORAL ASPECTS

Edited by

ALON CHEN

Academic Press is an imprint of Elsevier 125 London Wall, London EC2Y 5AS, United Kingdom 525 B Street, Suite 1650, San Diego, CA 92101, United States 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom Copyright © 2020 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-12-813983-7 For information on all Academic Press publications visit our website at https://www.elsevier.com/books-and-journals Publisher: Nikki Levy Acquisition Editor: Joslyn Chaiprasert-Paguio Editorial Project Manager: Kristi Anderson Production Project Manager: Prem Kumar Kaliamoorthi Cover Designer: Mark Rogers Typeset by TNQ Technologies Cover design by: Tali Wiesel, Graphic Designer, Design, Photography & Printing Branch, Research Services Division, Weizmann Institute of Science.

This book is dedicated in loving memory of Wylie W. Vale, a founder of mechanistic stress research and to whom I am forever indebted for his endless inspiration and encouragement. “Resilience is critical in all things. Grit and zest are qualities most predictive of success.” Wylie W. Vale (1941e2012)

v

Contributors Elisabeth B. Binder Department of Translational Research in Psychiatry, Max Planck Institute of Psychiatry, Munich, Germany; Department of Psychiatry and Behavioral Sciences and Department of Psychology, Emory University School of Medicine, Atlanta, GA, United States

Matthew Cranshaw University of Miami, Miller School of Medicine, Miami, FL, United States

Tracy L. Bale Department of Pharmacology, University of Maryland School of Medicine, Baltimore, MD, United States; Center for Epigenetic Research in Child Health and Brain Development, University of Maryland School of Medicine, Baltimore, MD, United States

E. Ron de Kloet Department of Endocrinology, Leiden University Medical Center, Leiden, The Netherlands

John F. Cryan Department of Anatomy and Neuroscience, University College Cork, Cork, Ireland; APC Microbiome Institute, University College Cork, Cork, Ireland

Jan M. Deussing Department of Stress Neurobiology and Behavioral Neurogenetics, Max Planck Institute of Psychiatry, Munich, Germany

Tallie Z. Baram Department of Anatomy/ Neurobiology, University of California-Irvine, Irvine, CA, United States; Department of Pediatrics, University of California-Irvine, Irvine, CA, United States

Olivia Engmann Laboratory of Neuroepigenetics, Brain Research Institute, Medical faculty of the University of Zurich and Institute for Neuroscience, Department of Health Science and Technology, ETH Zurich, Switzerland

David André Barrière Physiopathologie des Maladies Psychiatriques, UMR_S 894 Inserm, Centre de Psychiatrie et Neurosciences, Paris, France; Faculté de Médecine Paris Descartes, Service Hospitalo-Universitaire, Centre Hospitalier Sainte-Anne, Paris, France

C. Neill Epperson Department of Psychiatry, University of Colorado School of Medicine, Aurora, CO, United States Edward Ganz Life and Health Sciences Research Institute (ICVS), School of Medicine, University of Minho, Braga, Portugal; ICVS/ 3B’s, PT Government Associate Laboratory, Braga/Guimarães, Portugal

Jessica L. Bolton Department of Anatomy/ Neurobiology, University of California-Irvine, Irvine, CA, United States; Department of Pediatrics, University of California-Irvine, Irvine, CA, United States

Jakob Hartmann McLean Hospital e Harvard Medical School, Mailman Research Center, Neurobiology of Fear Laboratory, Belmont, MA, United States

Mallory E. Bowers Department of Psychiatry, Icahn School of Medicine at Mount Sinai, New York, NY, United States

Marloes J.A.G. Henckens Department of Cognitive Neuroscience, Donders Institute for Brain, Cognition and Behaviour, Radboudumc, Nijmegen, The Netherlands

Dennis S. Charney Department of Psychiatry, Icahn School of Medicine at Mount Sinai, New York, NY, United States Alon Chen Department of Stress Neurobiology and Behavioral Neurogenetics, Max Planck Institute of Psychiatry, Munich, Germany; Department of Neurobiology, Weizmann Institute of Science, Rehovot, Israel

James P. Herman Department of Pharmacology and System Physiology, University of Cincinnati, Cincinnati, OH, United States

xi

xii

CONTRIBUTORS

Matthew N. Hill Hotchkiss Brain Institute, Cumming School of Medicine, University of Calgary, Calgary, AB, Canada S.B. Hill Division of Depression and Anxiety, McLean Hospital; Department of Psychiatry, Harvard Medical School, Belmont, MA, United States Brian M. Iacoviello Department of Psychiatry, Icahn School of Medicine at Mount Sinai, New York, NY, United States; Discovery and Translational Research, Click Therapeutics, Inc., New York, NY, United States Orna Issler Nash Family Department of Neuroscience and Friedman Brain Institute, Icahn School of Medicine at Mount Sinai, New York, NY, United States Thérèse M. Jay Physiopathologie des Maladies Psychiatriques, UMR_S 894 Inserm, Centre de Psychiatrie et Neurosciences, Paris, France; Faculté de Médecine Paris Descartes, Service Hospitalo-Universitaire, Centre Hospitalier Sainte-Anne, Paris, France Marian Joëls Department of Translational Neuroscience, UMC Utrecht Brain Center, University Medical Center Utrecht, University of Utrecht, Utrecht, The Netherlands; University of Groningen/University Medical Center Groningen, Groningen, The Netherlands C.D. King Division of Depression and Anxiety, McLean Hospital; Department of Psychiatry, Harvard Medical School, Belmont, MA, United States Stafford L. Lightman Bristol Medical School: Translational Health Sciences, University of Bristol, Bristol, United Kingdom Ekaterina Likhtik Hunter College, The Graduate Center, City University of New York, New York, NY, United States Zachary S. Lorsch Nash Family Department of Neuroscience and Friedman Brain Institute, Icahn School of Medicine at Mount Sinai, New York, NY, United States David M. Lyons Department of Psychiatry and Behavioral Sciences, Stanford University, Stanford, CA, United States

Ricardo Magalhães Life and Health Sciences Research Institute (ICVS), School of Medicine, University of Minho, Braga, Portugal; ICVS/ 3B’s, PT Government Associate Laboratory, Braga/Guimarães, Portugal Isabelle M. Mansuy Laboratory of Neuroepigenetics, Brain Research Institute, Medical faculty of the University of Zurich and Institute for Neuroscience, Department of Health Science and Technology, ETH Zurich, Switzerland Bruce S. McEwen Alfred E. Mirsky Professor Head, Harold and Margaret Milliken Hatch, Laboratory of Neuroendocrinology, The Rockefeller University, New York, NY, United States Sébastien Mériaux Neurospin, JOLIOT, CEA, Gif-sur-Yvette, France Laia Morató Laboratory of Behavioral Genetics, Brain Mind Institute, École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland Kathleen E. Morrison Department of Pharmacology, University of Maryland School of Medicine, Baltimore, MD, United States; Center for Epigenetic Research in Child Health and Brain Development, University of Maryland School of Medicine, Baltimore, MD, United States Iris Müller Department of Genetics & Molecular Neurobiology, Institute of Biology, Otto-vonGuericke University Magdeburg, Magdeburg, Germany; Department of Psychological Sciences, Purdue University, Indianapolis, IN, United States Charles B. Nemeroff Department of Psychiatry and Behavioral Sciences, Miller School of Medicine, Miami, FL, United States Eric J. Nestler Nash Family Department of Neuroscience and Friedman Brain Institute, Icahn School of Medicine at Mount Sinai, New York, NY, United States Olivia F. O’Leary Department of Anatomy and Neuroscience, University College Cork, Cork, Ireland; APC Microbiome Institute, University College Cork, Cork, Ireland

CONTRIBUTORS

Lilia Papst Department of Translational Research in Psychiatry, Max Planck Institute of Psychiatry, Munich, Germany Sachin Patel Departments of Psychiatry and Behavioral Sciences, Pharmacology, Molecular Physiology & Biophysics, and The Vanderbilt Brain Institute, Vanderbilt University Medical Center, Nashville, TN, United States

xiii

A.V. Seligowski Division of Depression and Anxiety, McLean Hospital; Department of Psychiatry, Harvard Medical School, Belmont, MA, United States

Science,

Annabel K. Short Department of Anatomy/ Neurobiology, University of California-Irvine, Irvine, CA, United States; Department of Pediatrics, University of California-Irvine, Irvine, CA, United States

K.J. Ressler Division of Depression and Anxiety, McLean Hospital; Department of Psychiatry, Harvard Medical School, Belmont, MA, United States

Nuno Sousa Life and Health Sciences Research Institute (ICVS), School of Medicine, University of Minho, Braga, Portugal; ICVS/3B’s, PT Government Associate Laboratory, Braga/ Guimarães, Portugal

Rony Paz Weizmann Rehovot, Israel

Institute

of

Gal Richter-Levin Department of Psychology, University of Haifa, Haifa, Israel; Sagol Department of Neurobiology, University of Haifa, Haifa, Israel; The Integrated Brain and Behavior Research Center (IBBR), University of Haifa, Haifa, Israel Mariana Rodrigues Life and Health Sciences Research Institute (ICVS), School of Medicine, University of Minho, Braga, Portugal; ICVS/ 3B’s, PT Government Associate Laboratory, Braga/Guimarães, Portugal; Algoritmi Centre, University of Minho, Braga, Portugal Carmen Sandi Laboratory of Behavioral Genetics, Brain Mind Institute, École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland R. Angela Sarabdjitsingh Department of Translational Neuroscience, UMC Utrecht Brain Center, University Medical Center Utrecht, University of Utrecht, Utrecht, The Netherlands Alan F. Schatzberg Department of Psychiatry and Behavioral Sciences, Stanford University, Stanford, CA, United States Mathias V. Schmidt Max Planck Institute of Psychiatry, Munich, Germany

Francesca Spiga Bristol Medical School: Translational Health Sciences, University of Bristol, Bristol, United Kingdom Oliver Stork Department of Genetics & Molecular Neurobiology, Institute of Biology, Otto-von-Guericke University Magdeburg, Magdeburg, Germany; Center for Behavioral Brain Sciences, Magdeburg, Germany Shariful A. Syed Department of Psychiatry and Behavioral Sciences, Miller School of Medicine, Miami, FL, United States Kuldeep Tripathi Sagol Department of Neurobiology, University of Haifa, Haifa, Israel Christiaan H. Vinkers VU University Medical Center, Amsterdam, The Netherlands A.P. Wingo Division of Depression and Anxiety, McLean Hospital; Department of Psychiatry, Harvard Medical School, Belmont, MA, United States Rachel Yehuda Department of Psychiatry, Icahn School of Medicine at Mount Sinai, New York, NY, United States; Mental Health Care Center, James J. Peters Veterans Affairs Medical Center, Bronx, NY, United States; Department of Neuroscience, Icahn School of Medicine at Mount Sinai, New York, NY, United States

About the editor Prof. Alon Chen is President-Elect of the Weizmann Institute of Science and will begin his term on December 1, 2019. He was Head of the Department of Neurobiology from 2016 to 19. He is also a Director and Scientific Member at the Max Planck Institute of Psychiatry, Munich, Germany, and serves as the Head of the Max Planck SocietydWeizmann Institute of Science Laboratory for Experimental Neuropsychiatry and Behavioral Neurogenetics. He is an adjunct professor at the Medical School of the Ludwig Maximilian University, Munich. Prof. Chen received a BSc in Biological Studies, with distinction, from Ben-Gurion University in 1995, and a PhD from the Weizmann Institute of Science in 2001 (Direct PhD Program, with distinction). During his PhD studies, Prof. Chen also received an MBA from Ben-Gurion University. He was a postdoctoral fellow at the Salk Institute for Biological Studies in California, where he started researching stress. In 2005, he joined the faculty of the Weizmann Institute, in the Department of Neurobiology. At the Weizmann Institute, he is the incumbent of the Vera and John Schwartz Family Professorial Chair. Prof. Chen’s research focuses on the neurobiology of stress, particularly the mechanisms by which the brain regulates the response to stressful challenges and how this response is linked to psychiatric disorders. The collective long-term goal of his research is to elucidate the pathways and mechanisms by which stressors are perceived, processed, and transduced into neuroendocrine and behavioral responses under healthy and pathological conditions. His laboratory has made significant discoveries in this field, including fundamental aspects of the organism’s stress response and actions that link specific stress-related genes, epigenetic mechanisms, and brain circuits with anxiety disorders, depression, eating disorders, and the metabolic syndrome. Prof. Chen and his team use both genetic mouse models and human patients to ultimately create the scientific groundwork for therapeutic interventions to treat stress-related behavioral and physiological disorders. Prof. Chen is known for his excellent communication and interpersonal skills, strong leadership aptitude, and the ability to identify opportunities and to convert challenges into innovative solutions.

xv

Preface “It is not stress that kills us, it is our reaction to it.” Hans Selye (1907e1982)

Given that exposure to stressful life experiences is often unavoidable, understanding what makes individuals resilient to stress and how resilience can be built is of great interest and constitutes an integral part of preventive and therapeutic efforts. Since proresilient molecular and cellular mechanisms can counteract the deleterious effects of stressful challenges or trauma, a better understanding of resilience promoting factors and processes, as well as inter-individual differences in resilience is needed. Consequently, this understanding could pave the way for new clinical interventions for stressrelated psychopathologies including anxiety, depression and post-traumatic stress disorder (PTSD). To date, the field of stress neurobiology has largely recognized stress resilience as simply the absence of any psychopathology after an extremely stressful event or chronic stress exposure. This book aims to provide a broader and comprehensive overview of stress resilience and presents it rather as a highly complex process of effective and adaptive coping to diverse stressful or traumatic stimuli; a continuum of adjustable physiological and behavioral changes that shows a large inter-individual variation and can change over time. World-leading scientists and psychiatrists, working in the newly emerging field of stress resilience, present and discuss the diverse biological and environmental factors that shape and determine

an individual’s response to stressful stimuli, manifested with a susceptible or resilient outcome. Both humans and animals show notable variability in their responses to stressful challenges. In some individuals, a major acute insult or chronic stressor triggers abnormal behavioral and physiological responses and precipitates the onset of psychiatric disease, while in most others, the same stressors have little to no effect. This phenomenon of individual resistance to stressors has been broadly termed stress resilience. Commonly, stress resilience, which is essential for good health and wellbeing, refers to the individual’s ability to retain a set of adaptive characteristics that enable coping and recovery from stressful challenges or trauma or even enable one to thrive as result of this process. The collective and pivotal aim of the organism’s response to stressors is the maintenance of homeostasis in the presence of real or perceived challenges. This process requires numerous adaptive responses involving changes in the central nervous and neuroendocrine systems. When a situation is perceived as stressful, the brain activates many neuronal circuits, linking centers involved in sensory, motor, autonomic, neuroendocrine, cognitive, and emotional functions in order to adapt to the demand posed by the threat. However, the details of the pathways by which the brain translates stressful stimuli into the final, integrated biological response are only partially

xvii

xviii

PREFACE

understood. Nevertheless, there is extensive evidence showing that inappropriate regulation, disproportional intensity, or chronic and/or irreversible activation of the stress response is linked to the etiology and pathophysiology of an array of physiological and behavioral disorders. Previously, most research focused on understanding what positions an individual at greater risk for developing stress-related disorders, but more recently the focus has shifted to those individuals who do not develop significant psychopathology following stress, and who are typically referred to as being resilient. In several animal models and in human studies, resilience is associated with rapid activation of the stress response and its efficient termination. It is further characterized by the capacity to constrain stress-induced increases in corticotropin-releasing factor (CRF) and cortisol through an elaborate negative feedback system. Stress mediators, such as noradrenaline, the CRF family of neuropeptides, endocannabinoids or corticosterone/cortisol, are of obvious significance for understanding the mechanism of resilience. A proper balance in signaling cascades that regulate physiological responses and behavioral adaptation to a stressor is key in understanding the mechanisms of resilience. Thus, for optimal resilience, the sympathetic and parasympathetic nervous system, the pro- and antiinflammatory cytokines, and the activating and inhibiting arms of the hypothalamicpituitary-adrenal (HPA) axis need to be in balance. Resilience may be demonstrated by resistance to the negative effects of stressful challenges or by recovery to a normal state of functioning more quickly than expected following a stressful event. As such, it is important to distinguish between resistance to, and recovery from, stressful events, as these outcomes may involve distinct brain

regions, neurochemical processes, and unique biomarkers. Some consider stress resilience to be a preexisting personality trait, independent of risk exposure. Trait characteristics and assessments suggest that cognitive capabilities, personality, and neurobiological factors work alongside environmental factors to make certain individuals more or less resilient. Experiences that are emotionally stressful but not traumatic promote coping and build resilience since they are known to enhance learning and memory mechanisms, and can be used in therapeutic settings to foster recovery and resilience. Thus, rather than just a lack of significant psychological symptoms, we can also define resilience by the specific mechanisms that help to reduce one’s risk of developing such symptoms. For example, while there is no single genetic marker that predicts the development of PTSD following trauma, there do appear to be biological markers, mechanisms, and processes that help buffer the effects of trauma. The unique characteristics of resilient individuals have gained substantial interest in recent years, and growing efforts in animal models have attempted to unravel the molecular and cellular mechanisms that underlie this phenomenon. These animal studies have identified changes in several molecules, pathways and circuits, which involve multiple brain regions. While some molecular pathways identified in resilience overlap with those regulated in the opposite direction in stress susceptibility, others are unique to stress-resilient individuals. Again, this suggests that the molecular and cellular basis of resilience is not merely the absence of susceptibility, but rather active and adaptive processes, with genetic, epigenetic, transcriptional, cellular and circuit ebased mechanisms playing important roles in mediating the behavioral and physiological response to stressful challenges.

PREFACE

In addition to the neurobiological factors associated with resilience, psychosocial factors are thought to play a critical role. These factors comprise cognitive and behavioral traits such as optimism, cognitive flexibility, active coping skills, social support networks, physical activity, and a personal moral compass. Resilience promoting factors include having caring and supportive relationships, good communication and problem-solving skills, and the ability to manage strong feelings and impulses. The mechanisms underlying resilience may be primed early on in life by the interplay of environmental factors, including the quality of care-giving and the degree of adversity experienced, together with genetic factors that impact the regulation of the stress response, which in turn may influence the development of brain circuits relevant for emotion regulation. Stress habituation, which involves prior exposure to manageable stressors, reduces the behavioral and physiological responses to later stressors. Prior exposure to stressors increases the sense of control of stressful situations, and through desensitization reduces the amount of negative emotions experienced when confronted with these situations again, i.e., it teaches one how to respond to stressors more effectively. A predisposition to emotional and cognitive disorders originates early in life. The concepts of gene-environment interactions, and the importance of early-life experience for later resilience or vulnerability, have been demonstrated in both animal models and human populations. Early-life experience can modulate vulnerability vs. resilience to emotional and cognitive disorders in adulthood. This suggests that early-life is a particularly sensitive period, during which beneficial or adverse events can cause a later propensity towards stress resilience or vulnerability. Most early-life experiences are

xix

generated from signals received from the primary caregiver, and perturbations in these signals can program stress-related behaviors. These caregiver signals cause lasting changes in brain circuitry and function, including in networks associated with learning and memory, and with emotional and stress responses. Underlying these changes in brain circuits are transcriptional and epigenetic mechanisms. These molecular changes set in motion a signaling cascade that can determine an individual’s resilience or vulnerability to stress later on in life. The interaction of genetic factors with environmental stressors shapes the developing brain towards susceptibility or resilience. By studying gene-environment interactions, we can gain understanding of resilience mechanisms and information on relevant molecular and cellular mechanisms, brain circuits and behavioral strategies. A detailed map of gene-environment interactions in large longitudinal cohorts with repeated biological, neuroimaging, behavioral and symptomatic measures may allow us to dissect mechanisms of resilience at different developmental stages or even across generations and suggest strategies for enhancing resilience. From studies in animal models, a wide range of molecular and cellular changes have been associated with stress resilience. In particular, studies have identified resilientspecific changes at the levels of RNA, protein, chromatin, and DNA, all of which can have an impact on neuronal function and affect circuit-level interactions within the brain. Many studies of resilience to date have examined effects on candidate genes or molecular pathways known to be perturbed in stress-susceptible animals or in humans with depression. However, an increasing number of studies are the result of unbiased genomewide profiling approaches. Such approaches, when combined with advanced systems

xx

PREFACE

biology and bioinformatics analysis, have the potential to reveal novel regulators of stress resilience. For these cases, in vivo validation is essential to provide causal evidence that a given target molecule is indeed pro-resilient. In this book, we also touch on the growing evidence of the interplay between the peripheral system and the brain in the context of stress resilience. We suggest potential mechanisms and treatment approaches for stress-related disorders, which extend beyond the brain, and therefore our focus should not be limited to targeting CNS mechanisms. Another important aspect of resilience is the role that sex plays, as many psychiatric disorders are both linked to exposure to stressful life events and are sex-biased in symptomology or prevalence. There are profound sex differences in stress-induced psychiatric disorders, with females being more likely than males to develop these diseases. However, most of the mechanistic studies to date have focused exclusively on males. Sex is a key factor in determining when an individual might be vulnerable to stress, what type of stress is likely to produce long-term negative consequences, and in which behavioral domains the stressinduced dysfunction will manifest. Although the last decade has been very fruitful in terms of elaborating the molecular and cellular changes that define stress resilience, additional mechanistic studies are still greatly needed. The molecular basis of stress resilience is complex, with numerous brain regions and enumerable molecular, biochemical and cellular mediators involved. Although resilience appears to restore control-like behavior, multiple studies point

to active processes occurring at the molecular and cellular levels, which compensate for, bypass, or overcome the harmful effects of stressors. Delineating the complex interactions between the genetic, epigenetic, and environmental factors promoting stress resilience is essential for understanding this complex phenomenon, and will allow the development of preventative therapeutic approaches that aim to enhance, build or train resilience in at-risk populations. As outlined and discussed throughout the book, stress resilience is a highly complex process of adaptive coping to diverse stressful or traumatic stimuli; it is a continuum of physiological and behavioral modifications that shows large inter-individual variation and can change over time. Considering the frequency and range of stressors and traumatic experiences humans can face, it is essential to recognize the factors that contribute to resilience versus other outcomes, including the emergence of psychiatric disorders. Understanding these factors will help us to promote resilience in individuals before they encounter stress or trauma, and can inform us on the best treatment for individuals facing stress or struggling with trauma. This book aims to describe this complex phenomenon and present the latest knowledge on the molecular and behavioral aspects of stress resilience. Alon Chen, Vera and John Schwartz Family Professorial Chair Weizmann Institute of Science, Rehovot, Israel and Max Planck Institute of Psychiatry, Munich, Germany

Acknowledgments I would like to profoundly thank my colleagues in the field of stress research who selflessly agreed to contribute to this book and without whom this book would not have been possible. Many thanks go to Jessica Keverne for her support in editing. I would also like to thank the graphic design team at the Weizmann Institute of Science, especially Tali Wiesel for the cover image and Ishai Sher, Genia Brodsky and Keren Katzav for their dedicated work on reformatting the chapter figures. Finally, my thanks go to the team at Elsevier for bringing the book from a concept into print.

xxi

C H A P T E R

1

A life-course, epigenetic perspective on resilience in brain and body Bruce S. McEwen Alfred E. Mirsky Professor Head, Harold and Margaret Milliken Hatch, Laboratory of Neuroendocrinology, The Rockefeller University, New York, NY, United States

Introduction Resilience can be defined as “the ability to achieve a successful outcome in the face of adversity.” Understanding what this means in biological terms requires an understanding of “epigenetics,” as it is now applied to gene-environment interactions as well as the realization that the brain and body are continually changing as the life course proceeds and that one cannot simply “roll back the clock.” Moreover, positive and negative events during the life course, including events before conception and during gestation, can have long-term effects. Furthermore, the plasticity of the adult as well as developing brain provides the ability of experiences to change trajectories in a positive or negative direction. The mediators of this plasticity include not only endogenous neurotransmitters and neuromodulators but also mechanisms involving the cell surface, the cell nucleus, and mitochondria, along with circulating steroid and metabolic hormones. Two-way interactions between brain and body are of paramount significance, and the concepts of allostasis and allostatic load emphasize that the same mediators that promote adaptation in a biphasic and nonlinear manner can also promote pathophysiology when they are overused or dysregulated among themselves. We begin by considering the meaning of “stress” and the concepts of allostasis and allostatic load before introducing brain adaptive plasticity and epigenetics and how they produce adaptive as well as maladaptive plasticity. Studies of gene expression show that the brain is continually changing and that one cannot “roll back the clock.” Moreover, experiences can have lasting positive influences as in successful attachment or lasting negative influences as in early life abuse and neglect or traumatic events that cause PTSD. This raises the issue of interventions where we now must refer to “resilience” rather than “reversal” in describing what appears to be “recovery” and must therefore think about the process as a “redirection” of a trajectory in more positive direction.

Stress Resilience https://doi.org/10.1016/B978-0-12-813983-7.00001-X

1

Copyright © 2020 Elsevier Inc. All rights reserved.

2

1. A life-course, epigenetic perspective on resilience in brain and body

What is stress? “Stress” is a widely used and ambiguous word and so this chapter will use the following classifications of types of stress: good stress, tolerable stress, and toxic stress. See http://developingchild.harvard.edu/library/reports_and_working_papers/policy_ framework/ for paper related to toxic stress. "Good stress" is a term used in popular language to refer to the experience of rising to a challenge, taking a risk, and feeling rewarded by an often-positive outcome. A related term is “eustress.” Good self-esteem and good impulse control and decision-making capability, all functions of a healthy architecture of the brain, are important here! Even adverse outcomes can be “growth experiences” for individuals with such positive, adaptive characteristics that promote resilience in the face of adversity. “Tolerable stress” refers to those situations where bad things happen, but the individual with healthy brain architecture is able to cope, often with the aid of family, friends, and other individuals who provide support. These adverse outcomes can be “growth experiences” for individuals with such positive, adaptive characteristics and support systems that promote resilience. Here, “distress” refers to the uncomfortable feeling related to the nature of the stressor and the degree to which the individual feels a lack of ability to influence or control the stressor (Lazarus and Folkman, 1984; Diez Roux and Mair, 2010; Theall et al., 2013). Finally, “toxic stress” refers to the situation in which bad things happen to an individual who has limited support and who may also have brain architecture that reflects effects of adverse early life events that have impaired the development of good impulse control and judgment and adequate self-esteem. Here, the degree and/or duration of “distress” may be greater. With toxic stress, the inability to cope is likely to have adverse effects on behavior and physiology, and this will result in a higher degree of allostatic overload, as will be explained later in this chapter.

Definition of stress, allostasis, and allostatic load In spite of the further definitions of types of stress, the word “stress” is still an ambiguous term and has connotations that make it less useful in understanding how the body handles events that are stressful. Insight into these processes can lead to a better understanding of how best to intervene, a topic that will be discussed at the end of this chapter. There are two sides to this story: on the one hand, the body responds to almost any event or challenge, whether or not we call it “stress,” by releasing chemical mediatorsdfor example, catecholamines that increase heart rate and blood pressuredand helps us cope with the situation; on the other hand, chronic elevation of these same mediatorsdfor example, chronically increased heart rate and blood pressuredproduces a chronic wear and tear on the cardiovascular system that can result, over time, in disorders such as strokes and heart attacks. For this reason, the term “allostasis” was introduced by Sterling and Eyer in 1988 to refer to the active process by which the body responds to daily events and maintains homeostasis (allostasis literally means “achieving stability through change”). Since chronically increased allostasis can lead to disease, we introduced the term “allostatic load or overload” to refer to the wear and tear that results from either too much stress or the

Protection and damage as the two sides of the response to experiences

3

FIGURE 1.1

Central role of the brain in allostasis and the behavioral and physiological response to stressors. Modified from McEwen, B.S. 1998. Protective and damaging effects of stress mediators. The New England Journal of Medicine 338, 171e179, with permission.

inefficient management of allostasis, for example, not turning off the response when it is no longer needed. Other forms of allostatic load involve not turning on an adequate response in the first place or not habituating to the recurrence of the same stressor and thus dampening the allostatic response (McEwen, 1998). See Fig. 1.1.

Protection and damage as the two sides of the response to experiences Protection via allostasis and wear and tear on the body and brain via allostatic load/overload are the two contrasting sides of the physiology involved in defending the body against the challenges of daily life. Besides adrenalin and noradrenalin, there are many mediators that participate in allostasis, and they are linked together in a network of regulation that is nonlinear, meaning that each mediator has the ability to regulate the activity of the other mediators, sometimes in a biphasic manner (McEwen, 2006). Glucocorticoid produced by the adrenal cortex in response to ACTH from the pituitary gland is the other major “stress hormone.” Pro- and antiinflammatory cytokines are produced by many cells in the body, and they regulate each other and are, in turn, regulated by glucocorticoids and catecholamines. Whereas catecholamines can increase proinflammatory cytokine production, glucocorticoids are known to inhibit this production. And yet, there are exceptionsd proinflammatory effects of glucocorticoids that depend on dose and cell or tissue type (Dinkel et al., 2003; Frank et al., 2012). The parasympathetic nervous system also plays an important regulatory role in this nonlinear network of allostasis, as it generally opposes

4

1. A life-course, epigenetic perspective on resilience in brain and body

the sympathetic nervous system and, for example, slows the heart and also has antiinflammatory effects (Borovikova et al., 2000; Sloan et al., 2007). What this nonlinearity and interaction among mediators means is that when any one mediator is increased or decreased, there are compensatory changes in the other mediators that depend on time course and level of change of each of the mediators. Unfortunately, we cannot measure all components of this system simultaneously, and we must, therefore, rely on measurements of only a few of them in any one study. Yet the nonlinearity must be kept in mind in interpreting the results. A good example of the biphasic actions of stress, that is, “protection versus damage,” is in the immune system, in which an acute stressor activates an acquired immune response via mediation by catecholamines and glucocorticoids and locally produced immune mediators; yet, a chronic exposure to the same stressor over several weeks has the opposite effect and results in immune suppression (Dhabhar, 2009; Dhabhar et al., 2012). Acute stresseinduced immune enhancement is good for enhancing immunization, fighting an infection, or repairing a wound, but it is deleterious to health for an autoimmune condition such as psoriasis or Krohn’s disease. On the other hand, immune suppression is good in the case of an autoimmune disorder and deleterious for fighting an infection or repairing a wound. In an immune sensitive skin cancer, acute stress is effective in inhibiting tumor progression, whereas chronic stress exacerbates progression (Dhabhar et al., 2010; Saul et al., 2005). Other experiences such as loneliness and social isolation (Cacioppo et al., 2011) or living in an ugly, noisy, or chaotic environment (Evans and Wachs, 2010) alter these same mediators and can lead to allostatic load and overload (Evans et al., 2007) (see Table 1.1). The same applies to poor or inadequate sleep and circadian disruption as in jet lag and shift work (McEwen and Karatsoreos, 2015). Other health-damaging behaviors also contribute to allostatic load/overload, including smoking, alcohol in excess, diet and amount of food, and lack of physical activity (Seeman et al., 2010). Finally, because the mediators of allostasis and allostatic load/overload affect virtually the whole body at the same time, it should not be surprising that there is poly- or multimorbidity TABLE 1.1

Conditions/experiences that “get under the skin” and dysregulate physiology.

All have effects whether or not called “stress” Loneliness Lack of social support Circadian disruption: jet lag, shift work Ugly, noise, polluted neighborhood; lack of green space Health behaviors Lack of physical activity Diet: quality and quantity of food Sleep Alcohol Smoking

Brain as the central organ of allostasis and allostatic load/overload

5

of diseases and disorders (Tomasdottir et al., 2015). For example, the association of diabetes and insulin resistance with depression and increased risk for dementia points to a common pathophysiology in which inflammation and glutamatergic hyperactivity play a key role (Rasgon and McEwen, 2016).

Brain as the central organ of allostasis and allostatic load/overload Plasticity and vulnerability of the hippocampus The hippocampus was the first higher brain center that was recognized as a target of adrenal steroids (McEwen et al., 1968), and it has figured prominently as a gateway to our understanding of how stress impacts neural architecture and behavior in adult as well as developing brains. The hippocampus expresses both type I mineralocorticoid (MR) and type II glucocorticoid (GR) receptors (Reul and DeKloet, 1985), and these receptors mediate a biphasic response to adrenal steroids in the CA1 region, although only facilitation in the dentate gyrus (Joels, 2006). Yet, the hippocampus, nevertheless, shows a diminished excitability in the absence of adrenal steroids (Margineanu et al., 1994). Other brain regions, such as the paraventricular nucleus, lacking in MR but having GR, show a monophasic negative response to increasing glucocorticoid levels (Joels, 2006). Adrenal steroids exert biphasic effects on excitability of hippocampal neurons in terms of long-term potentiation and primed burst potentiation (Diamond et al., 1992; Pavlides et al., 1994, 1995a,b) and show parallel biphasic effects on memory (Pugh et al., 1997; Okuda et al., 2004). A form of structural plasticity is the remodeling of dendrites in the hippocampus, as well as in the amygdala and prefrontal cortex (McEwen and Gianaros, 2011). In hippocampus, chronic restraint stress (CRS), daily for 21 days, causes retraction and simplification of dendrites in the CA3 region of the hippocampus (McEwen, 1999; Sousa et al., 2000). Such dendritic reorganization is found in both dominant and subordinate rats undergoing adaptation to psychosocial stress in the visible burrow system, and it is independent of adrenal size (McKittrick et al., 2000). It also occurs in psychosocial stress in intruder tree shrews in a residentdintruder paradigm, with a time course of 28 days (Magarinos et al., 1996), a procedure that does not cause a loss of pyramidal neurons in the hippocampus (Vollmann-Honsdorf et al., 1997). The mossy fiber input to the CA3 region in the stratum lucidum appears to drive the dendritic remodeling, leading to the retraction of the apical dendrites above this input (McEwen, 1999). Moreover, the thorny excrescence giant spines, on which the mossy fiber terminals form their synapses, show stress-induced modifications (Stewart et al., 2005). And the number of active synaptic zones between thorny excrescences and mossy fiber terminals is rapidly modulated during hibernation and recovery from the hibernating state (Magarinos et al., 2006). The thorny excrescences are not the only spines affected by CRS. Dendritic spines also show remodeling, with increased spine density reported after CRS on apical dendrites of CA3 neurons (Sunanda Rao and Raju, 1995) and decreased spine density reported for CA1 pyramidal neurons (Magarinos et al., 2010). Indeed the entire mossy fiber-thorny excrescence complex decreases in volume with stress and increases in volume with environmental enrichment (Stewart et al., 2005).

6 TABLE 1.2

1. A life-course, epigenetic perspective on resilience in brain and body

Cell surface and nucleocytoplasmic interaction that are necessary/permissive for remodeling.

PSA-NCAMdcell surface “antistickiness” (Sandi, Rutishauser, McCall, Weil) Endonuclease N removes PSA from NCAM; dendrite expansion Facilitate plasticity but also limits it Cell adhesion molecules: neuroligin-2; nectin-3 (van der Kooij, Sandi) Chronic stress disrupts neuroligin-neurexin interaction Chronic stress reduces nectin-3 via MMP9 protease Nuclear pore complex NUP-62 (Kinoshita . Kohtz) Reduction leads to dendrite shrinkage Possibly due to nuclear cytoplasmic communication

Exploration of the underlying mechanism for this remodeling of dendrites and synapses reveals that it is not adrenal size nor presumed amount of physiological stress per se that determines dendritic remodeling, but rather a complex set of other factors that modulate neuronal structure (McEwen, 1999). Indeed, after repeated stress, dendritic remodeling recovers (Conrad et al., 1999), and in species of mammals that hibernate, dendritic remodeling is a reversible process and occurs within hours of the onset of hibernation in European hamsters and ground squirrels, and it is also reversible within hours of wakening of the animals from torpor (Magarinos et al., 2006; Popov and Bocharova, 1992; Popov et al., 1992; Arendt et al., 2003). Along with data on posttranslational modification of cytoskeletal proteins (Table 1.2), this implies that reorganization of the cytoskeleton is taking place rapidly and reversibly (Arendt et al., 2003) and that changes in dendrite length and branching are not damage but a form of adaptive structural plasticity.

Cellular processes involved in structural plasticity The neuronal surface, cytoskeleton, and nuclear envelope are each implicated in the mechanisms of stress-induced retraction and expansion of dendrites and synapse turnover. The polysialylated form of neural cell adhesion molecule (PSA-NCAM) is expressed in the CA3 and DG region of the hippocampus and is believed to denote the capacity for adaptive structural plasticity in many parts of the CNS (Seki and Arai, 1999; Rutishauser, 2008; Theodosis et al., 1999). Repeated stress causes retraction of CA3 hippocampal dendrites accompanied by a modest increase in PSA-NCAM expression, possibly the result of glucocorticoid mediation (Pham et al., 2003). Using EndoN to remove PSA from NCAM, Sandi (2004) reported impairment of consolidation of contextual fear conditioning. Using the same treatment, we reported considerable expansion of the dendritic tree in both CA3 and CA1 and a marked increase in excitotoxicity and damage to CA3 neurons; repeated stress still caused some dendrite retraction after PSA removal (McCall et al., 2013). Thus, although PSA-NCAM is a facilitator of plasticity, the PSA moiety appears to also limit the extent of dendritic growth and yet is not necessary for dendritic retraction under stress (see Table 1.2).

Brain as the central organ of allostasis and allostatic load/overload

7

Two other classes of cell adhesion molecules are reported to change with chronic stress, with behavioral consequences. Neuroligins (NLGNs) are important for proper synaptic formation and functioning and are critical regulators of the balance between neural excitation/inhibition (E/I), and CRS reduced hippocampal NLGN-2 levels, in association with reduced sociability and increased aggression (van der Kooij et al., 2014a; Wood et al., 2003). This occurred along with a reduction of NLGN-2 expression throughout the hippocampus, detectable in different layers of the CA1, CA3, and DG subfields. Intrahippocampal administration of neurolide-2 that interferes with the interaction between NLGN-2 and neurexin led to reduced sociability and increased aggression, thus mimicking effects of chronic stress (van der Kooij et al., 2014a). CRS also increases activity of matrix metalloproteinase-9 (MMP-9) in the CA1. MMP-9 carries out proteolytic processing of another cell adhesion molecule, nectin-3. Chronic stress reduced nectin-3 in the perisynaptic CA1, but not in the CA3, with consequences for social exploration, social recognition, and a CA1-dependent cognitive task. Implicated in this is a stress-related increase in extracellular glutamate and NMDA receptor mediation of MMP-9 (van der Kooij et al., 2014b). These findings are reminiscent of the CA1-specific effects of tissue plasminogen activator, mediating stress effects on spine density in CA1 (Pawlak et al., 2005) (see Table 1.2). Actin polymerization plays a key role in filopodial extension and spine synapse formation as well as plasticity within the synapse itself (Matus et al., 2000), and cytoskeletal remodeling is an important factor in the effects of stress and other environmental manipulations. Hibernation in European hamsters and ground squirrels results in rapid retraction of dendrites of CA3 pyramidal neurons and equally rapid expansion when hibernation torpor is reversed (Magarinos et al., 2006; Popov et al., 1992). The retraction of dendrites is accompanied by increases in a soluble phosphorylated form of tau that may indicate disruption of the cytoskeleton, which permits the dendrite shortening and possible protection from excitotoxicity; at the same time, PSA-NCAM expression is lost during hibernation torpor reducing the capacity for plasticity (Arendt et al., 2003). This model highlights the important role that tau plays in normal cytoskeletal function, a fact that should be emphasized when attempting to understand its role in pathology (Morris et al., 2011) (see Table 1.2). Even though dendrite retraction and regrowth would appear to involve a reversible depolymerization and repolymerization of the cytoskeleton, there are other processes that point to the importance of nuclear factors. A recent example is the unexpected role of a cell nuclear pore complex protein, NUP-62, in the stress-induced dendritic remodeling in the CA3 region of hippocampus (Kinoshita et al., 2014). First identified as a gene that was downregulated in the prefrontal cortex of depressed patients (Tochigi et al., 2008), NUP-62 was also found to be reduced in response to chronic stress in CA3 neurons of rodents (Kinoshita et al., 2014). Importantly, the levels of other nuclear pore complex genes were unchanged with chronic stress, supporting the specificity of its role in stress remodeling. Subsequent in vitro studies confirmed that the downregulation of NUP-62 is associated with dendritic retraction, and this effect is regulated at the molecular level by NUP-62 phosphorylation at a PYK2 site, which results in its retention in the cytoplasm (Kinoshita et al., 2014). A role of NUP-62 in maintaining chromatin structure for transcription is suggested as well as in nucleocytoplasmic transport (Kinoshita et al., 2014) (see Table 1.2).

8

1. A life-course, epigenetic perspective on resilience in brain and body

Extension of stress effects to amygdala and prefrontal cortex Besides the hippocampus, the amygdala and prefrontal cortex are targets of stress and display structural plasticity after both acute and chronic stress. Neurons in the basolateral amygdala (BLA) expand dendrites after chronic immobilization stress and increase spine density (Vyas et al., 2002), whereas neurons in medial amygdala show reduced spine density after chronic stress (Bennur et al., 2007). The latter changes are dependent on tissue plasminogen activator released by CRF (Matys et al., 2004), based on a tPA-ko mouse, whereas stress effects in BLA are not so dependent (Bennur et al., 2007). These stress-induced changes are accompanied by increases in anxiety-like behavior (Vyas et al., 2002; Pawlak et al., 2003) and suggest that stress causes a reorganization and dysfunction of circuits within the amygdala. Indeed, the medial amygdala is quite different and shows not only spine loss but also dendrite retraction after CRS, and this may underlie impaired social interactions of chronically stressed animals (Lau et al., 2017). Glucocorticoids and excitatory amino acids are involved in the mechanism for dendritic expansion in the BLA with chronic stress, along with BDNF (Lakshminarasimhan and Chattarji, 2012; McEwen and Chattarji, 2007) and, indeed, a single bolus of corticosterone mimics the effects of 10 days of chronic immobilization to cause BLA dendrite expansion (Mitra and Sapolsky, 2008). Overexpression of BDNF in mice increases dendritic length in both CA3 and BLA and occludes the effects of chronic stress to decrease dendritic branching in CA3 and increase it in BLA (Govindarajan et al., 2006). Without such overexpression, chronic stress causes a downregulation of BDNF in CA3 hippocampus and an upregulation of BDNF in the BLA, and the effect in BLA persists 21 days poststress, whereas that in CA3 has normalized (Lakshminarasimhan and Chattarji, 2012); moreover, acute stress with a 10day delay, which causes BLA to develop increased anxiety and increased density of spines in BLA neurons (Mitra et al., 2005), caused BDNF expression to rise and stay elevated for 10 days, whereas that in CA3 fell after acute stress but did so only transiently (Lakshminarasimhan and Chattarji, 2012). Corticosterone levels increased after both acute and chronic stress and remained elevated after chronic, but not after acute stress. Although some of the immediate consequences of stressdelevated glucocorticoids and glutamatedare similar in amygdala and hippocampus, they lead to contrasting patterns of BDNF expression and structural plasticity (see Table 1.3). This implies that signaling mechanisms more downstream of the initial changes in glucocorticoids and glutamate, but upstream of BDNF, may hold the key to the differential impact of stress in these brain areas. Importantly, BDNF infusion into the hippocampus of stressed rodents helped to protect against the deleterious effects of stress despite high levels of circulating corticosterone. This suggests that BDNF could be a final point of convergence for the stress-induced effects in the hippocampus. BDNF-mediated signaling is involved in stress response, but the direction and nature of signaling is region specific and stress specific and is influenced by epigenetic modifications along with posttranslational modifications (Gray et al., 2013; Lakshminarasimhan and Chattarji, 2012). Concurrently, with changes in the amygdala, neurons in the medial prefrontal cortex show reversible dendritic shrinkage after chronic stress (Radley et al., 2004, 2005), with spine loss (Radley et al., 2008), that can be inhibited by blocking NMDA receptors (Martin and Wellman, 2011), similar to stress-induced atrophy of neurons in the CA3 hippocampus (see above).

Brain as the central organ of allostasis and allostatic load/overload

TABLE 1.3

9

Secreted signaling molecules that are necessary/permissive for remodeling.

BDNF - Brain-derived neurotrophic factor (Francis Lee, Shona Chattarji) - Facilitator of plasticity or growth; floor and ceiling CRF - Corticotropin-releasing factor (Tallie Baram and colleagues) - Downregulates thin spines via RhoA signaling tPA - Tissue plasminogen activator (Sid Strickland, Robert Pawlak, Tomas Matys) - Required for stress-induced spine loss in CA1 hippocampus and medial amygdala - CRF regulates tPA release Lipocalin-2 - Secreted protein (Robert Pawlak) - Acute stress induces lipocalin-2 - Lipocalin-2 ko increases neuronal excitability and anxiety Endocannabinoids (Hill, Holmes, Hillard, Gorzalka) - Induced via glucocorticoids - Regulate emotionality and HPA habituation and shut off - Buffer against stress induced remodeling

This chronic stresseinduced atrophy is associated with deficits in executive function and cognitive flexibility (Liston et al., 2006; Dias-Ferreira et al., 2009), and the stressors that cause this to happen also include circadian disruption (Karatsoreos et al., 2011). Although medial prefrontal cortical neurons show atrophy with chronic stress, neurons in the orbitofrontal cortex show hypertrophy (Liston et al., 2006) similar to what happens in the BLA (Vyas et al., 2002). The consequences of these changes are increased anxiety and increased vigilance, both adaptive traits in a dangerous environment.

Other mediators of structural plasticity In addition to glucocorticoids, excitatory amino acids, BDNF, and tPA, other secreted signaling molecules play an important role in the remodeling of neural tissue during stress (see Table 1.3). Corticotropin-releasing factor (CRF), which is better known for its role in governing secretion of ACTH and glucocorticoids, plays a key role in stress-induced dendritic remodeling in the CA1 region of the hippocampus (Pawlak et al., 2005; Chen et al., 2006). Linking CRF with tPA discussed above, there is evidence that in the amygdala tPA release is stimulated by CRF (Matys et al., 2004). Similarly, lipocalin-2 is a novel modulator of spine plasticity with different effects in amygdala and hippocampus (Mucha et al., 2011; Skrzypiec et al., 2013). Acute stress increases lipocalin-2 levels, and lipocalin-2 downregulates mushroom spines and generally inhibits actin motility in hippocampus. Remarkably, deletion of lipocalin-2 increases neuronal excitability and anxiety, and, in amygdala, the absence of lipocalin-2 increases the basal number of spines and prevents a stress-induced increase in spine density (Mucha et al., 2011; Skrzypiec et al., 2013). Endocannabinoids are another class of signaling molecules that importantly regulate multiple aspects of the stress response. In addition to contributing to the termination (Hill et al., 2011a) of the acute response to stress, as well as habituation to repeated stress (Hill et al., 2010), endocannabinoids also appear to be important for the regulation of structural plasticity under conditions of repeated stress. For example, cannabinoid 1 (CB1) receptoredeficient

10

1. A life-course, epigenetic perspective on resilience in brain and body

mice exhibit reductions in prefrontal cortical dendritic length and complexity, while having enhanced and more complex dendritic arbors within the BLA, which parallels the effects of chronic stress (Lee et al., 2014; Hill et al., 2011b). More importantly, chronic stress and corticosterone treatment are both known to impair endocannabinoid signaling at multiple levels, through both a downregulation of the CB1 receptor (Hill et al., 2005) and a reduction in the levels of the endocannabinoid, anandamide, mediated by an increase in its hydrolysis by the enzyme fatty acid amide hydrolase (FAAH) (Hill et al., 2013; Bowles et al., 2012). Given the parallels between genetic deletion of the CB1 receptor and the ability of chronic stress to impair endocannabinoid signaling, it is interesting to note that elevating anandamide/CB1 receptor signaling, through genetic or pharmacological impairment of FAAH, retards the ability of chronic stress to produce dendritic hypertrophy in the BLA as well as concomitant changes in emotional behavior (Hill et al., 2013; Lomazzo et al., 2015; Bortolato et al., 2007; Rossi et al., 2010; Gunduz-Cinar et al., 2013). Collectively, these data indicate that endocannabinoid signaling buffers against many of the effects of stress and appears to be important for limiting the effects of chronic stress on structural plasticity within these identified limbic circuits. At a mechanistic level, this is likely due to the ability of CB1 receptor signaling to gate glutamatergic release, as it has been shown that CB1 receptoredeficient mice exhibit greater changes in glutamatergic signaling and excitotoxicity within the PFC following chronic stress (Zoppi et al., 2011). Moreover, similar to the protective effects of CB1 receptor activation identified within the amygdala, administration of a CB1 receptor agonist during repeated stress can reduce the increase in glutamatergic signaling, the induction of proinflammatory cytokines, and lipid peroxidation within the PFC (Zoppi et al., 2011). As such, the release of endocannabinoids during stress may temper changes in structural plasticity by limiting the magnitude of glutamate release in response to stress, and under conditions of chronic stress, when this system becomes compromised, the loss of this endogenous buffer facilitates excess glutamate release and the ensuing changes in dendritic morphology. Linking this model with the previously described factors, it is interesting to note that in addition to promoting tPA release, CRF has also been found to induce anandamide hydrolysis by FAAH (Gray et al., 2015), suggesting the possibility that CRF could act as an orchestrator of multiple signaling molecules, all of which converge in structural changes within the brain following chronic stress.

Glucocorticoids as key players in PTSD vulnerability Recent studies have suggested that blood-based biomarkers may be able to predict aspects of brain signaling associated with trauma-related effects in both males and females, specifically with respect to convergence onto GR signaling pathways. After a predatorscent-stress (PSS) exposure, male and female rats were classified into vulnerable (i.e., “PTSD-like”) and resilient (i.e., minimally affected) phenotypes on the basis of their performance on a variety of behavioral measures (Daskalakis et al., 2014). Using genome-wide expression profiling in blood, amygdala, and hippocampus, glucocorticoid signaling was the only convergent pathway associated with individual differences in susceptibility.

Lessons of an ever-changing brain from gene expression

11

Moreover, corticosterone treatment 1 h after PSS exposure prevented anxiety and hyperarousal 7 days later in both sexes, consistent with prior findings in the same as well as in another PTSD animal model (Zohar et al., 2011; Rao et al., 2012), confirming GR involvement in sequelae of traumatic stress.

Sex differences Animal models of stress effects on the brain show that females and males response differently to acute and chronic stressors because of developmental factors involving both epigenetic effects of hormones along with genes in the sex chromosomes themselves (McCarthy and Arnold, 2011). One of the important discoveries of the past several decades is that sex hormones have effects throughout the brain. Indeed, sexually mature female rats do not show dendritic retraction from CRS in hippocampus (Galea et al., 1997) although chronic stress over puberty of immature male and female rats produces qualitatively similar structural plasticity in hippocampus, amygdala, and prefrontal cortex (Eiland et al., 2012). Chronically stressed adult female rats actually show enhanced memory function, whereas chronically stressed males are impaired (Luine, 2002). Sex differences in the brain are subtle but widespread (McEwen and Milner, 2017) and yet males and females do many things equally well: for example, in human subjects, taking tests on empathy, men and women do equally well, but the brain activation patterns during the tests show different brain regions are activated (Derntl et al., 2010). This is reminiscent of an animal model study in which, despite no overall sex differences in fear conditioning freezing behavior, the neural processes underlying successful or failed extinction maintenance are sex specific (Gruene et al., 2015). Given other work showing sex differences in stress-induced structural plasticity in prefrontal cortex projections to amygdala and other cortical areas (Shansky et al., 2010), these findings are relevant not only to sex differences in fear conditioning and extinction but also, according to Gruene et al., “also to exposure-based clinical therapies, which are similar in premise to fear extinction and which are primarily used to treat disorders that are more common in women than in men” (Gruene et al., 2015).

Lessons of an ever-changing brain from gene expression The hippocampus has been an important gateway to understanding the effects of glucocorticoids and stress on gene expression. Recent advances in technology have allowed for high-throughput analysis of gene expression changes in response to stress (Rubin et al., 2014). For example, microarray analysis of whole hippocampus after acute and chronic stress, as well as recovery from stress in mice, has revealed a number of insights surrounding stress-induced neuroplasticity (Gray et al., 2014). Although acute stress and chronic stress modulate a core set of genes, there are numerous expression changes that are exclusive to each condition, highlighting how the duration and intensity of stress alters reactivity.

12

1. A life-course, epigenetic perspective on resilience in brain and body

Furthermore, corticosterone injections did not yield the same expression profile as acute stress, suggesting that in vivo stressors are able to activate a diverse set of pathways independent of GR activation. Finally, characterization of expression profiles after an extended recovery from chronic stress (21d) revealed that, despite a normalization of anxiety-related behaviors, recovery did not represent a return to the stress-naïve baseline but rather represented a new state in which reactivity to a novel stressor produced a unique expression profile (Gray et al., 2014). Studies in rats have also confirmed that gene expression profiles can vary significantly from the immediate end of stress (1 h) to 24 h after the end of stress (Wang et al., 2010) and that chronic stress can alter the transcriptional response to an acute corticosterone injection in dentate gyrus (Datson et al., 2013). These studies demonstrate that a history of stress exposure can have a lasting impact on future stress reactivity and hippocampal function. Many of the genes identified as changed after chronic stress by Datson and DeKloet are known epigenetic regulators, providing one possible mechanism underlying the persistent alterations in the expression response beyond the end of stress exposure.

Epigenetics: two meanings that are both important for prevention and treatment “Epigenetics” now refers to events “above the genome” that regulate expression of genetic information without altering the DNA sequence. Besides the CpG methylation (Szyf et al., 2005), other mechanisms include histone modifications that repress or activate chromatin unfolding (Allfrey, 1970) and the actions of noncoding RNAs (Mehler, 2008), as well as transposons and retrotransposons (Griffiths and Hunter, 2014) and RNA editing (Mehler and Mattick, 2007). Again, the hippocampus is providing important information. For example, Reul and colleagues have shown that the forced swimming-induced behavioral immobility response requires histone H3 phosphoacetylation and c-Fos induction in distinct dentate granule neurons through recruitment of the NMDA/ERK/MSK 1/2 pathway (Chandramohan et al., 2008). Another histone mark changed in hippocampus, most prominently in the dentate gyrus, is the dramatic induction by an acute restraint stress of trimethylation of lysine 9 on histone H3, which is associated with repression of a number of retrotransposon elements and reduction of the coding and noncoding RNA normally produced by the repressed DNA (Hunter et al., 2009). This repression is lost with repeated stress, suggesting the possibility that those retrotransposon elements may impair genomic stability under conditions of chronic stress (Hunter et al., 2015). A current practical application is the search for rapidly acting antidepressants because classical antidepressants work very slowly and are not effective on every depressed individual. Epigenetic processes are likely involved in the chronic relapsing nature of major depression, the strikingly higher incidence of depression in women after puberty, the high discordance rates between monozygotic twins, as well as the individual responsivity to stress that precipitates mood-related behaviors in susceptible individuals. In the course of these studies, we are learning more about epigenetic mechanisms that connect EAA function

Individual differences and experiences throughout the life course

13

with neural remodeling and stress-related behavior. The identification of the fast antidepressant effects of ketamine, an NMDA receptor blocker, has resulted in a paradigm shift toward the discovery of a new generation of rapidly acting antidepressants (Li et al., 2010). Recently, our laboratory and other groups have found that the naturally occurring compound acetyl-L-carnitine (LAC) shows fast antidepressant efficacy in genetic and environmentally induced animal models of depression through the epigenetic modulation of the metabotropic glutamate receptor, mGlu2, in the hippocampus (Nasca et al., 2013; Cuccurazzu et al., 2013). mGlu2 is known to exert an inhibitory tone on glutamate release from synapses, and pharmacological modulators for this receptor are under clinical development to treat stress-related mood disorders, such as anxiety and depression (Nicoletti et al., 2015). Using the same animal models, 14 days of treatment with the tricyclic antidepressant clomipramine were needed to promote antidepressant responses, which disappeared when the treatment was stopped. In contrast, LAC antidepressant effects were still evident after 2 weeks of drug withdrawal (Nasca et al., 2013). The persistent effects of LAC suggested the involvement of stable molecular adaptations that may be reflected at the level of histone modifications in controlling mGlu2 transcription in the hippocampus. Indeed, LAC increases levels of mGlu2 receptors by acetylation of the histone H3K27 among other mechanisms (discussed below). These findings support previous studies that have shown that histone deacetylase (HDAC) inhibitors, given intraperitoneally, normalize gene expression profiles in vulnerable brain regions, such as hippocampus, amygdala, and nucleus accumbens, to promote fast antidepressant responses following stress (Covington et al., 2011; Tsankova et al., 2006). The use of agents such as LAC that act on histone remodeling to regulate transcription of the mGlu2 gene offers alternative and complementary strategies to ketamine and HDACs’ inhibitors with safer profiles and lower potential for drug dependence (Nicoletti et al., 2015).

Individual differences and experiences throughout the life course In the course of this work, we have become aware of individual differences among inbred mice and rats (Cavigelli and McClintock, 2003; Miller et al., 2012; Freund et al., 2013). Using a simple light-dark test to rapidly screen naïve mice (Nasca et al., 2015), we found that a subset of mice shows elevated hippocampal MR levels and that this baseline difference makes those mice with higher MR show greater stress-induced reduction in mGlu2 accompanied by more anxiety and depressive-like behaviors. How MR activation does this is not yet clear, but it activates a mechanism that is opposite to that of LAC, which has been shown to use the acetyltransferase P300 to acetylate lysine 27 on histone H3 (Nasca et al., 2013). Likewise, the nature of the experiences of the animals that develop higher MR is also not yet known but may involve maternal care and stressors in the neonatal nesting environment (Francis et al., 1999). The epigenetic allostasis model points to a developmental origin of individual differences in the responses to stress and implies that unknown early-life epigenetic influences program each individual to different trajectories of behavioral and physiological responses to later stressful life events. In line with this model, previous studies have also associated increased hippocampal MR levels in juvenile animals with anxiety-like behavior in adulthood (Brydges et al., 2014; Korte et al., 1995).

14

1. A life-course, epigenetic perspective on resilience in brain and body

Early-life experiences Early-life experiences carry an even greater weight in terms of how an individual reacts to new situations. Early-life physical and sexual abuse carries with it a lifelong burden of behavioral and pathophysiological problems (Felitti et al., 1998). Cold and uncaring families produce long-lasting emotional problems in children (Repetti et al., 2002). Some of these effects are seen on brain structure and function and in the risk for later depression and posttraumatic stress disorder (Shonkoff et al., 2009). One of the biological consequences of early life adversity is the prolonged elevation of inflammatory cytokines as well as poor dental health, obesity, elevated blood pressure in children and young adults (Danese and McEwen, 2012). Harsh language is among the components of early-life adversity and has been shown to increase inflammatory markers (Miller and Chen, 2010). The physical environment makes a huge difference, with crowding, noise, and ugliness, along with physical danger, which are a major contributor to allostatic overload both during development and throughout adult life (Diez Roux and Mair, 2010; Chang et al., 2009; Evans et al., 2005). Animal models have been useful in providing insights into behavioral and physiological mechanisms. Individual differences in anxiety-like behaviors are evident (Cavigelli and McClintock, 2003; Nasca et al., 2015). Early-life maternal care in rodents is a powerful determinant of lifelong emotional reactivity and stress hormone reactivity, and increases in both are associated with earlier cognitive decline and a shorter life span (Meaney et al., 1991). Effects of early maternal care are transmitted across generations by the subsequent behavior of the female offspring, as they become mothers, and methylation of DNA on key genes appears to play a role in this epigenetic transmission (Francis et al., 1999; Meaney and Szyf, 2005). Yet, the mother is not the sole determinant of offspring emotional and physical development but rather modulates it by her behavior toward the infant, particularly in the immediate aftermath of infant experiences of novelty inside or outside of the home cage (Tang et al., 2014). Furthermore, in rodents, abuse of the young is associated with an attachment, rather than an avoidance, of the abusive mother, an effect that increases the chances that the infant can continue to obtain food and other support until weaning (Moriceau and Sullivan, 2006). Moreover, other conditions that affect the rearing process can also affect emotionality in offspring. For example, uncertainty in the food supply for rhesus monkey mothers leads to increased emotionality in offspring and possibly an earlier onset of obesity and diabetes (Coplan et al., 2001; Kaufman et al., 2007). Besides the important role of the social and physical environment and experiences of individuals in the health outcomes, genetic factors also play an important role. Different alleles of commonly occurring genes determine how individuals will respond to experiences. For example, the short form of the serotonin transporter is associated with a number of conditions such as alcoholism, and individuals who have this allele are more vulnerable to respond to stressful experiences by developing depressive illness (Caspi et al., 2003; Spinelli et al., 2012). In childhood, individuals with an allele of the monoamine oxidase A gene are more vulnerable to abuse in childhood and more likely to themselves become abusers and to show antisocial behaviors compared with individuals with another commonly occurring

References

15

allele (Caspi et al., 2002). Nevertheless, in a positive, nurturing environment, as formulated by Suomi and by Tom Boyce and colleagues (Suomi, 2006; Obradovic et al., 2010; Boyce and Ellis, 2005), these same alleles may lead to successful outcomes, which has led them to be called “reactive or context-sensitive alleles” rather than “bad genes.”

Intervention For prevention and treatment, in the spirit of integrative medicine, it is important to let the “wisdom of the body” prevail and to focus upon strategies that center around the use of targeted behavioral therapies along with treatments, including pharmaceutical agents, that “open up windows of plasticity” in the brain and facilitate the efficacy of the behavioral interventions (McEwen, 2012). This is because a major challenge throughout the life course is to find ways of redirecting future behavior and physiology in more positive and healthy directions (Halfon et al., 2014). In keeping with the original definition of epigenetics (Waddington, 1942) as the emergence of characteristics not previously evident or even predictable from an earlier developmental stage (e.g., think about a fertilized frog or human egg which look similar and what happens as each develop!), we do not mean “reversibility” as in “rolling back the developmental clock” but rather “redirection” as well as “resilience,” which can be defined as “achieving a successful outcome in the face of adversity.”

References Allfrey, V.G., 1970. Changes in chromosomal proteins at times of gene activation. Federation Proceedings 29, 1447e1460. Arendt, T., Stieler, J., Strijkstra, A.M., et al., 2003. Reversible paired helical filament-like phosphorylation of tau is an adaptive process associated with neuronal plasticity in hibernating animals. Journal of Neuroscience 23, 6972e6981. Bennur, S., Shankaranarayana Rao, B.S., Pawlak, R., Strickland, S., McEwen, B.S., Chattarji, S., 2007. Stress-induced spine loss in the medial amygdala is mediated by tissue-plasminogen activator. Neuroscience 144, 8e16. Borovikova, L.V., Ivanova, S., Zhang, M., et al., 2000. Vagus nerve stimulation attenuates the systemic inflammatory response to endotoxin. Nature 405, 458e462. Bortolato, M., Mangieri, R.A., Fu, J., et al., November 15, 2007. Antidepressant-like activity of the fatty acid amide hydrolase inhibitor URB597 in a rat model of chronic mild stress. Biological Psychiatry 62 (10), 1103e1110. Bowles, N.P., Hill, M.N., Bhagat, S.M., Karatsoreos, I.N., Hillard, C.J., McEwen, B.S., March 1, 2012. Chronic, noninvasive glucocorticoid administration suppresses limbic endocannabinoid signaling in mice. Neuroscience 204, 83e89. Boyce, W.T., Ellis, B.J., Spring 2005. Biological sensitivity to context: I. An evolutionary-developmental theory of the origins and functions of stress reactivity. Development and Psychopathology 17 (2), 271e301. Brydges, N.M., Jin, R., Seckl, J., Holmes, M.C., Drake, A.J., Hall, J., January 2014. Juvenile stress enhances anxiety and alters corticosteroid receptor expression in adulthood. Brain and Behavior 4 (1), 4e13. Cacioppo, J.T., Hawkley, L.C., Norman, G.J., Berntson, G.G., August 2011. Social isolation. Annals of the New York Academy of Sciences 1231, 17e22. Caspi, A., McClay, J., Moffitt, T.E., et al., 2002. Role of genotype in the cycle of violence in maltreated children. Science 297, 851e854. Caspi, A., Sugden, K., Moffitt, T.E., et al., 2003. Influence of life stress on depression: moderation by a polymorphism in the 5-HTT gene. Science 301, 386e389. Cavigelli, S.A., McClintock, M.K., 2003. Fear of novelty in infant rats predicts adult corticosterone dynamics and an early death. Proceedings of the National Academy of Sciences of the United States of America 100, 16131e16136.

16

1. A life-course, epigenetic perspective on resilience in brain and body

Chandramohan, Y., Droste, S.K., Arthur, J.S., Reul, J.M., May 2008. The forced swimming-induced behavioural immobility response involves histone H3 phospho-acetylation and c-Fos induction in dentate gyrus granule neurons via activation of the N-methyl-D-aspartate/extracellular signal-regulated kinase/mitogen- and stressactivated kinase signalling pathway. European Journal of Neuroscience 27 (10), 2701e2713. Chang, V.W., Hillier, A.E., Mehta, N.K., June 1, 2009. Neighborhood racial isolation, disorder and obesity. Social Forces: a Scientific Medium of Social Study and Interpretation 87 (4), 2063e2092. Chen, Y., Fenoglio, K.A., Dube, C.M., Grigoriadis, D.E., Baram, T.Z., 2006. Cellular and molecular mechanisms of hippocampal activation by acute stress are age-dependent. Molecular Psychiatry 11, 992e1002. Conrad, C.D., Magarinos, A.M., LeDoux, J.E., McEwen, B.S., 1999. Repeated restraint stress facilitates fear conditioning independently of causing hippocampal CA3 dendritic atrophy. Behavioral Neuroscience 113, 902e913. Coplan, J.D., Smith, E.L.P., Altemus, M., et al., 2001. Variable foraging demand rearing: sustained elevations in cisternal cerebrospinal fluid corticotropin-releasing factor concentrations in adult primates. Biological Psychiatry 50, 200e204. Covington 3rd, H.E., Vialou, V.F., LaPlant, Q., Ohnishi, Y.N., Nestler, E.J., April 15, 2011. Hippocampal-dependent antidepressant-like activity of histone deacetylase inhibition. Neuroscience Letters 493 (3), 122e126. Cuccurazzu, B., Bortolotto, V., Valente, M.M., et al., October 2013. Upregulation of mGlu2 receptors via NF-kappaB p65 acetylation is involved in the proneurogenic and antidepressant effects of acetyl-L-carnitine. Neuropsychopharmacology 38 (11), 2220e2230. Danese, A., McEwen, B.S., April 12, 2012. Adverse childhood experiences, allostasis, allostatic load, and age-related disease. Physiology and Behavior 106 (1), 29e39. Daskalakis, N.P., Cohen, H., Cai, G., Buxbaum, J.D., Yehuda, R., September 16, 2014. Expression profiling associates blood and brain glucocorticoid receptor signaling with trauma-related individual differences in both sexes. Proceedings of the National Academy of Sciences of the United States of America 111 (37), 13529e13534. Datson, N.A., van den Oever, J.M., Korobko, O.B., Magarinos, A.M., de Kloet, E.R., McEwen, B.S., September 2013. Previous history of chronic stress changes the transcriptional response to glucocorticoid challenge in the dentate gyrus region of the male rat hippocampus. Endocrinology 154 (9), 3261e3272. Derntl, B., Finkelmeyer, A., Eickhoff, S., et al., January 2010. Multidimensional assessment of empathic abilities: neural correlates and gender differences. Psychoneuroendocrinology 35 (1), 67e82. Dhabhar, F.S., 2009. Enhancing versus suppressive effects of stress on immune function: implications for immunoprotection and immunopathology. Neuroimmunomodulation 16 (5), 300e317. Dhabhar, F.S., Saul, A.N., Daugherty, C., Holmes, T.H., Bouley, D.M., Oberyszyn, T.M., January 2010. Short-term stress enhances cellular immunity and increases early resistance to squamous cell carcinoma. Brain, Behavior, and Immunity 24 (1), 127e137. Dhabhar, F.S., Malarkey, W.B., Neri, E., McEwen, B.S., September 2012. Stress-induced redistribution of immune cellsefrom barracks to boulevards to battlefields: a tale of three hormoneseCurt Richter Award winner. Psychoneuroendocrinology 37 (9), 1345e1368. Diamond, D.M., Bennett, M.C., Fleshner, M., Rose, G.M., 1992. Inverted-U relationship between the level of peripheral corticosterone and the magnitude of hippocampal primed burst potentiation. Hippocampus 2, 421e430. Dias-Ferreira, E., Sousa, J.C., Melo, I., et al., 2009. Chronic stress causes frontostriatal reorganization and affects decision-making. Science 325, 621e625. Diez Roux, A.V., Mair, C., February 2010. Neighborhoods and health. Annals of the New York Academy of Sciences 1186, 125e145. Dinkel, K., MacPherson, A., Sapolsky, R.M., 2003. Novel glucocorticoid effects on acute inflammation in the CNS. Journal of Neurochemistry 84, 705e716. Eiland, L., Ramroop, J., Hill, M.N., Manley, J., McEwen, B.S., January 2012. Chronic juvenile stress produces corticolimbic dendritic architectural remodeling and modulates emotional behavior in male and female rats. Psychoneuroendocrinology 37 (1), 39e47. Evans, G.W., Wachs, T.D., 2010. Chaos and its Influence on Children’s Development: An Ecological Perspective, first ed. American Psychological Association, Washington, DC. Evans, G.W., Gonnella, C., Marcynyszyn, L.A., Gentile, L., Salpekar, N., July 2005. The role of chaos in poverty and children’s socioemotional adjustment. Psychological Science 16 (7), 560e565. Evans, G.W., Kim, P., Ting, A.H., Tesher, H.B., Shannis, D., 2007. Cumulative risk, maternal responsiveness, and allostatic load among young adolescents. Development and Psychopathology 43, 341e351.

References

17

Felitti, V.J., Anda, R.F., Nordenberg, D., et al., 1998. Relationship of childhood abuse and household dysfunction to many of the leading causes of death in adults. The adverse childhood experiences (ACE) study. American Journal of Preventive Medicine 14, 245e258. Francis, D., Diorio, J., Liu, D., Meaney, M.J., 1999. Nongenomic transmission across generations of maternal behavior and stress responses in the rat. Science 286, 1155e1158. Frank, M.G., Thompson, B.M., Watkins, L.R., Maier, S.F., February 2012. Glucocorticoids mediate stress-induced priming of microglial pro-inflammatory responses. Brain, Behavior, and Immunity 26 (2), 337e345. Freund, J., Brandmaier, A.M., Lewejohann, L., et al., May 10, 2013. Emergence of individuality in genetically identical mice. Science 340 (6133), 756e759. Galea, L.A.M., McEwen, B.S., Tanapat, P., Deak, T., Spencer, R.L., Dhabhar, F.S., 1997. Sex differences in dendritic atrophy of CA3 pyramidal neurons in response to chronic restraint stress. Neuroscience 81, 689e697. Govindarajan, A., Rao, B.S.S., Nair, D., et al., 2006. Transgenic brain-derived neurotrophic factor expression causes both anxiogenic and antidepressant effects. Proceedings of the National Academy of Sciences of the United States of America 103, 13208e13213. Gray, J.D., Milner, T.A., McEwen, B.S., June 3, 2013. Dynamic plasticity: the role of glucocorticoids, brain-derived neurotrophic factor and other trophic factors. Neuroscience 239, 214e227. Gray, J.D., Rubin, T.G., Hunter, R.G., McEwen, B.S., November 2014. Hippocampal gene expression changes underlying stress sensitization and recovery. Molecular Psychiatry 19 (11), 1171e1178. Gray, J.M., Vecchiarelli, H.A., Morena, M., et al., March 4, 2015. Corticotropin-releasing hormone drives anandamide hydrolysis in the amygdala to promote anxiety. Journal of Neuroscience 35 (9), 3879e3892. Griffiths, B.B., Hunter, R.G., September 5, 2014. Neuroepigenetics of stress. Neuroscience 275, 420e435. Gruene, T.M., Roberts, E., Thomas, V., Ronzio, A., Shansky, R.M., 2015 Aug 1. Sex-specific neuroanatomical correlates of fear expression in prefrontal-amygdala circuits. Biological Psychiatry 78 (3), 186e193. https://doi.org/ 10.1016/j.biopsych.2014.11.014. Epub 2014 Nov 29. PMID: 25579850. Gunduz-Cinar, O., Hill, M.N., McEwen, B.S., Holmes, A., November 2013. Amygdala FAAH and anandamide: mediating protection and recovery from stress. Trends in Pharmacological Sciences 34 (11), 637e644. Halfon, N., Larson, K., Lu, M., Tullis, E., Russ, S., February 2014. Lifecourse health development: past, present and future. Maternal and Child Health Journal. 18 (2), 344e365. Hill, M.N., Patel, S., Carrier, E.J., et al., 2005. Downregulation of endocannabinoid signaling in the hippocampus following chronic unpredictable stress. Neuropsychopharmacology 30, 508e515. Hill, M.N., McLaughlin, R.J., Bingham, B., et al., May 18, 2010. Endogenous cannabinoid signaling is essential for stress adaptation. Proceedings of the National Academy of Sciences of the United States of America 107 (20), 9406e9411. Hill, M.N., McLaughlin, R.J., Pan, B., et al., July 20, 2011. Recruitment of prefrontal cortical endocannabinoid signaling by glucocorticoids contributes to termination of the stress response. Journal of Neuroscience 31 (29), 10506e10515. Hill, M.N., Hillard, C.J., McEwen, B.S., September 2011. Alterations in corticolimbic dendritic morphology and emotional behavior in cannabinoid CB1 receptor-deficient mice parallel the effects of chronic stress. Cerebral Cortex 21 (9), 2056e2064. Hill, M.N., Kumar, S.A., Filipski, S.B., et al., October 2013. Disruption of fatty acid amide hydrolase activity prevents the effects of chronic stress on anxiety and amygdalar microstructure. Molecular Psychiatry 18 (10), 1125e1135. Hunter, R.G., McCarthy, K.J., Milne, T.A., Pfaff, D.W., McEwen, B.S., 2009. Regulation of hippocampal H3 histone methylation by acute and chronic stress. Proceedings of the National Academy of Sciences of the United States of America 106, 20912e20917. Hunter, R.G., Gagnidze, K., McEwen, B.S., Pfaff, D.W., 2015. Stress and the dynamic genome: steroids, epigenetics, and the transposome. Proceedings of the National Academy of Sciences of the United States of America 112, 6828e6833. Joels, M., 2006. Corticosteroid effects in the brain: U-shape it. Trends in Pharmacological Sciences 27, 244e250. Karatsoreos, I.N., Bhagat, S., Bloss, E.B., Morrison, J.H., McEwen, B.S., January 25, 2011. Disruption of circadian clocks has ramifications for metabolism, brain, and behavior. Proceedings of the National Academy of Sciences of the United States of America. 108 (4), 1657e1662. Kaufman, D., Banerji, M.A., Shorman, I., et al., 2007. Early-life stress and the development of obesity and insulin resistance in juvenile bonnet macaques. Diabetes 56, 1e5.

18

1. A life-course, epigenetic perspective on resilience in brain and body

Kinoshita, Y., Hunter, R.G., Gray, J.D., et al., November 11, 2014. Role for NUP62 depletion and PYK2 redistribution in dendritic retraction resulting from chronic stress. Proceedings of the National Academy of Sciences of the United States of America 111 (45), 16130e16135. Korte, S.M., de Boer, S.F., de Kloet, E.R., Bohus, B., 1995. Anxiolytic-like effects of selective mineralocorticoid and glucocorticoid antagonists on fear-enhanced behavior in the elevated plus-maze. Psychoneuroendocrinology 20 (4), 385e394. Lakshminarasimhan, H., Chattarji, S., 2012. Stress leads to contrasting effects on the levels of brain derived neurotrophic factor in the hippocampus and amygdala. PLoS One 7 (1), e30481. Lau, T., Bigio, B., Zelli, D., McEwen, B.S., Nasca, C., 2017. Stress-induced structural plasticity of medial amygdala stellate neurons and rapid prevention by a candidate antidepressant. Molecular Psychiatry 22, 227e234. Lazarus, R.S., Folkman, S. (Eds.), 1984. Stress, Appraisal and Coping. Springer Verlag, New York. Lee, T.T., Filipski, S.B., Hill, M.N., McEwen, B.S., September 1, 2014. Morphological and behavioral evidence for impaired prefrontal cortical function in female CB1 receptor deficient mice. Behavioural Brain Research 271, 106e110. Li, N., Lee, B., Liu, R.J., et al., August 20, 2010. mTOR-dependent synapse formation underlies the rapid antidepressant effects of NMDA antagonists. Science 329 (5994), 959e964. Liston, C., Miller, M.M., Goldwater, D.S., et al., 2006. Stress-induced alterations in prefrontal cortical dendritic morphology predict selective impairments in perceptual attentional set-shifting. Journal of Neuroscience 26, 7870e7874. Lomazzo, E., Bindila, L., Remmers, F., et al., January 2015. Therapeutic potential of inhibitors of endocannabinoid degradation for the treatment of stress-related hyperalgesia in an animal model of chronic pain. Neuropsychopharmacology 40 (2), 488e501. Luine, V., 2002. Sex differences in chronic stress effects on memory in rats. Stress: The International Journal on the Biology of Stress 5, 205e216. Magarinos, A.M., McEwen, B.S., Flugge, G., Fuchs, E., 1996. Chronic psychosocial stress causes apical dendritic atrophy of hippocampal CA3 pyramidal neurons in subordinate tree shrews. Journal of Neuroscience 16, 3534e3540. Magarinos, A.M., McEwen, B.S., Saboureau, M., Pevet, P., 2006. Rapid and reversible changes in intrahippocampal connectivity during the course of hibernation in European hamsters. Proceedings of the National Academy of Sciences of the United States of America 103, 18775e18780. Magarinos, A.M., Li, C.J., Gal Toth, J., et al., 2010. Effect of brain-derived neurotrophic factor haploinsufficiency on stress-induced remodeling of hippocampal neurons. Hippocampus 21, 253e264. Margineanu, D.-G., Gower, A.J., Gobert, J., Wulfert, E., 1994. Long-term adrenalectomy reduces hippocampal granule cell excitability in vivo. Brain Research Bulletin 33, 93e98. Martin, K.P., Wellman, C.L., October 2011. NMDA receptor blockade alters stress-induced dendritic remodeling in medial prefrontal cortex. Cerebral Cortex 21 (10), 2366e2373. Matus, A., Brinkhaus, H., Wagner, U., 2000. Actin dynamics in dendritic spines: a form of regulated plasticity at excitatory synapses. Hippocampus 10, 555e560. Matys, T., Pawlak, R., Matys, E., Pavlides, C., McEwen, B.S., S, S., 2004. Tissue plasminogen activator promotes the effects of corticotropin releasing factor on the amygdala and anxiety-like behavior. Proceedings of the National Academy of Sciences of the United States of America 101, 16345e16350. McCall, T., Weil, Z.M., Nacher, J., et al., March 2013. Depletion of polysialic acid from neural cell adhesion molecule (PSA-NCAM) increases CA3 dendritic arborization and increases vulnerability to excitotoxicity. Experimental Neurology 241, 5e12. McCarthy, M.M., Arnold, A.P., June 2011. Reframing sexual differentiation of the brain. Nature Neuroscience 14 (6), 677e683. McEwen, B.S., 1998. Protective and damaging effects of stress mediators. The New England Journal of Medicine 338, 171e179. McEwen, B.S., 1999. Stress and hippocampal plasticity. Annual Review of Neuroscience 22, 105e122. McEwen, B.S., 2006. Protective and damaging effects of stress mediators: central role of the brain. Dialogues in Clinical Neuroscience: Stress: The International Journal on the Biology of Stress 8, 367e381. McEwen, B.S., October 16, 2012. Brain on stress: how the social environment gets under the skin. Proceedings of the National Academy of Sciences of the United States of America 109 (Suppl. 2), 17180e17185.

References

19

McEwen, B.S., Chattarji, S., 2007. Neuroendocrinology of stress. In: Handbook of Neurochemistry and Molecular Neurobiology, third ed. ed. Springer-Verlag, New York, pp. 572e593. McEwen, B.S., Gianaros, P.J., February 18, 2011. Stress- and allostasis-induced brain plasticity. Annual Review of Medicine 62, 431e445. McEwen, B.S., Karatsoreos, I.N., March 2015. Sleep deprivation and circadian disruption: stress, allostasis, and allostatic load. Sleep Medicine Clinics 10 (1), 1e10. McEwen, B.S., Milner, T.A., January 2, 2017. Understanding the broad influence of sex hormones and sex differences in the brain. Journal of Neuroscience Research 95 (1e2), 24e39. McEwen, B.S., Weiss, J., Schwartz, L., 1968. Selective retention of corticosterone by limbic structures in rat brain. Nature 220, 911e912. McKittrick, C.R., Magarinos, A.M., Blanchard, D.C., Blanchard, R.J., McEwen, B.S., Sakai, R.R., 2000. Chronic social stress reduces dendritic arbors in CA3 of hippocampus and decreases binding to serotonin transporter sites. Synapse 36, 85e94. Meaney, M.J., Szyf, M., 2005. Environmental programming of stress responses through DNA methylation: life at the interface between a dynamic environment and a fixed genome. Dialogues in Clinical Neuroscience 7, 103e123. Meaney, M., Aitken, D., Bhatnagar, S., Sapolsky, R., 1991. Postnatal handling attenuates certain neuroendocrine, anatomical and cognitive dysfunctions associated with aging in female rats. Neurobiology of Aging 12, 31e38. Mehler, M.F., 2008. Epigenetic principles and mechanisms underlying nervous system functions in health and disease. Progress in Neurobiology 86, 305e341. Mehler, M.F., Mattick, J.S., 2007. Noncoding RNAs and RNA editing in brain development, functional diversification, and neurological disease. Physiological Reviews 87, 799e823. Miller, G.E., Chen, E., June 2010. Harsh family climate in early life presages the emergence of a proinflammatory phenotype in adolescence. Psychological Science 21 (6), 848e856. Miller, M.M., Morrison, J.H., McEwen, B.S., April 1, 2012. Basal anxiety-like behavior predicts differences in dendritic morphology in the medial prefrontal cortex in two strains of rats. Behavioural Brain Research 229 (1), 280e288. Mitra, R., Sapolsky, R.M., 2008. Acute corticosterone treatment is sufficient to induce anxiety and amygdaloid dendritic hypertrophy. Proceedings of the National Academy of Sciences of the United States of America 105, 5573e5578. Mitra, R., Jadhav, S., McEwen, B.S., Vyas, A., Chattarji, S., 2005. Stress duration modulates the spatiotemporal patterns of spine formation in the basolateral amygdala. Proceedings of the National Academy of Sciences of the United States of America 102, 9371e9376. Moriceau, S., Sullivan, R., 2006. Maternal presence serves as a switch between learning fear and attraction in infancy. Nature Neuroscience 8, 1004e1006. Morris, M., Maeda, S., Vossel, K., Mucke, L., May 12, 2011. The many faces of tau. Neuron 70 (3), 410e426. Mucha, M., Skrzypiec, A.E., Schiavon, E., Attwood, B.K., Kucerova, E., Pawlak, R., November 8, 2011. Lipocalin-2 controls neuronal excitability and anxiety by regulating dendritic spine formation and maturation. Proceedings of the National Academy of Sciences of the United States of America. 108 (45), 18436e18441. Nasca, C., Xenos, D., Barone, Y., et al., March 19, 2013. L-acetylcarnitine causes rapid antidepressant effects through the epigenetic induction of mGlu2 receptors. Proceedings of the National Academy of Sciences of the United States of America 110 (12), 4804e4809. Nasca, C., Bigio, B., Zelli, D., Nicoletti, F., McEwen, B.S., June 2015. Mind the gap: glucocorticoids modulate hippocampal glutamate tone underlying individual differences in stress susceptibility. Molecular Psychiatry 20 (6), 755e763. Nicoletti, F., Bruno, V., Ngomba, R.T., Gradini, R., Battaglia, G., February 2015. Metabotropic glutamate receptors as drug targets: what’s new? Current Opinion in Pharmacology 20C, 89e94. Obradovic, J., Bush, N.R., Stamperdahl, J., Adler, N.E., Boyce, W.T., Jan-Feb. Biological sensitivity to context: the interactive effects of stress reactivity and family adversity on socioemotional behavior and school readiness. Child Development 81 (1), 270e289. Okuda, S., Roozendaal, B., McGaugh, J.L., 2004. Glucocorticoid effects on object recognition memory require trainingassociated emotional arousal. Proceedings of the National Academy of Sciences of the United States of America 101, 853e858. Pavlides, C., Kimura, A., Magarinos, A.M., McEwen, B.S., 1994. Type I adrenal steroid receptors prolong hippocampal long-term potentiation. NeuroReport 5, 2673e2677.

20

1. A life-course, epigenetic perspective on resilience in brain and body

Pavlides, C., Kimura, A., Magarinos, A.M., McEwen, B.S., 1995. Hippocampal homosynaptic long-term depression/ depotentiation induced by adrenal steroids. Neuroscience 68, 379e385. Pavlides, C., Watanabe, Y., Magarinos, A.M., McEwen, B.S., 1995. Opposing role of adrenal steroid Type I and Type II receptors in hippocampal long-term potentiation. Neuroscience 68, 387e394. Pawlak, R., Magarinos, A.M., Melchor, J., McEwen, B., Strickland, S., February 2003. Tissue plasminogen activator in the amygdala is critical for stress-induced anxiety-like behavior. Nature Neuroscience 6 (2), 168e174. Pawlak, R., Rao, B.S.S., Melchor, J.P., Chattarji, S., McEwen, B., Strickland, S., 2005. Tissue plasminogen activator and plasminogen mediate stress-induced decline of neuronal and cognitive functions in the mouse hippocampus. Proceedings of the National Academy of Sciences of the United States of America 102, 18201e18206. Pham, K., Nacher, J., Hof, P.R., McEwen, B.S., February 2003. Repeated restraint stress suppresses neurogenesis and induces biphasic PSA-NCAM expression in the adult rat dentate gyrus. European Journal of Neuroscience 17 (4), 879e886. Popov, V.I., Bocharova, L.S., 1992. Hibernation-induced structural changes in synaptic contacts between mossy fibres and hippocampal pyramidal neurons. Neuroscience 48, 53e62. Popov, V.I., Bocharova, L.S., Bragin, A.G., 1992. Repeated changes of dendritic morphology in the hippocampus of ground squirrels in the course of hibernation. Neuroscience 48, 45e51. Pugh, C.R., Tremblay, D., Fleshner, M., Rudy, J.W., 1997. A selective role for corticosterone in contextual-fear conditioning. Behavioral Neuroscience 111, 503e511. Radley, J.J., Sisti, H.M., Hao, J., et al., 2004. Chronic behavioral stress induces apical dendritic reorganization in pyramidal neurons of the medial prefrontal cortex. Neuroscience 125, 1e6. Radley, J.J., Rocher, A.B., Janssen, W.G.M., Hof, P.R., McEwen, B.S., Morrison, J.H., 2005. Reversibility of apical dendritic retraction in the rat medial prefrontal cortex following repeated stress. Experimental Neurology 196, 199e203. Radley, J.J., Rocher, A.B., Rodriguez, A., et al., March 1, 2008. Repeated stress alters dendritic spine morphology in the rat medial prefrontal cortex. The Journal of Comparative Neurology 507 (1), 1141e1150. Rao, R.P., Anilkumar, S., McEwen, B.S., Chattarji, S., September 15, 2012. Glucocorticoids protect against the delayed behavioral and cellular effects of acute stress on the amygdala. Biological Psychiatry 72 (6), 466e475. Rasgon, N.L., McEwen, B.S., 2016. Insulin resistance-a missing link no more. Molecular Psychiatry 21, 1648e1652. Repetti, R.L., Taylor, S.E., Seeman, T.E., 2002. Risky families: family social environments and the mental and physical health of offspirng. Psychological Bulletin 128, 330e366. Reul, J.M., DeKloet, E.R., 1985. Two receptor systems for corticosterone in rat brain: microdistribution and differential occupation. Endocrinology 117, 2505e2511. Rossi, S., De Chiara, V., Musella, A., et al., August 2010. Preservation of striatal cannabinoid CB1 receptor function correlates with the antianxiety effects of fatty acid amide hydrolase inhibition. Molecular Pharmacology 78 (2), 260e268. Rubin, T.G., Gray, J.D., McEwen, B.S., November 2014. Experience and the ever-changing brain: what the transcriptome can reveal. BioEssays: News and Reviews in Molecular, Cellular and Developmental Biology 36 (11), 1072e1081. Rutishauser, U., 2008. Polysialic acid in the plasticity of the developing and adult vertebrate nervous system. Nature Reviews Neuroscience 9, 26e35. Sandi, C., 2004. Stress, cognitive impairment and cell adhesion molecules. Nature Reviews Neuroscience 5, 917e930. Saul, A.N., Oberyszyn, T.M., Daugherty, C., et al., 2005. Chronic stress and susceptibility to skin cancer. Journal of the National Cancer Institute 97, 1760e1767. Seeman, T., Epel, E., Gruenewald, T., Karlamangla, A., McEwen, B.S., 2010. Socio-economic differentials in peripheral biology: cumulative allostatic load. Annals of the New York Academy of Sciences 1186, 223e239. Seki, T., Arai, Y., July 19, 1999. Different polysialic acid-neural cell adhesion molecule expression patterns in distinct types of mossy fiber boutons in the adult hippocampus. The Journal of Comparative Neurology 410 (1), 115e125. Shansky, R.M., Hamo, C., Hof, P.R., Lou, W., McEwen, B.S., Morrison, J.H., November 2010. Estrogen promotes stress sensitivity in a prefrontal cortex-amygdala pathway. Cerebral Cortex 20 (11), 2560e2567. Shonkoff, J.P., Boyce, W.T., McEwen, B.S., 2009. Neuroscience, molecular biology, and the childhood roots of health disparities. Journal of the American Medical Association 301, 2252e2259. Skrzypiec, A.E., Shah, R.S., Schiavon, E., et al., 2013. Stress-induced lipocalin-2 controls dendritic spine formation and neuronal activity in the amygdala. PLoS One 8 (4), e61046.

References

21

Sloan, R.P., McCreath, H., Tracey, K.J., Sidney, S., Liu, K., Seeman, T., 2007. RR interval variability is inversely related to inflammatory markers: the CARDIA study. Molecular Medicine 13, 178e184. Sousa, N., Lukoyanov, N.V., Madeira, M.D., Almeida, O.F.X., Paula-Barbosa, M.M., 2000. Reorganization of the morphology of hippocampal neurites and synapses after stress-induced damage correlates with behavioral improvement. Neuroscience 97, 253e266. Spinelli, S., Schwandt, M.L., Lindell, S.G., et al., February 2012. The serotonin transporter gene linked polymorphic region is associated with the behavioral response to repeated stress exposure in infant rhesus macaques. Development and Psychopathology 24 (1), 157e165. Stewart, M.G., Davies, H.A., Sandi, C., et al., 2005. Stress suppresses and learning induces plasticity in CA3 of rat hippocampus: a three-dimensional ultrastructural study of thorny excrescences and their postsynaptic densities. Neuroscience 131, 43e54. Sunanda Rao, M.S., Raju, T.R., 1995. Effect of chronic restraint stress on dendritic spines and excrescences of hippocampal CA3 pyramidal neurons e a quantitative study. Brain Research 694, 312e317. Suomi, S.J., December 2006. Risk, resilience, and gene x environment interactions in rhesus monkeys. Annals of the New York Academy of Sciences 1094, 52e62. Szyf, M., Weaver, I.C.G., Champagne, F.A., Diorio, J., Meaney, M.J., 2005. Maternal programming of steroid receptor expression and phenotype through DNA methylation in the rat. Frontiers in Neuroendocrinology 26, 139e162. Tang, A.C., Reeb-Sutherland, B.C., Romeo, R.D., McEwen, B.S., April 2014. On the causes of early life experience effects: evaluating the role of mom. Frontiers in Neuroendocrinology 35 (2), 245e251. Theall, K.P., Brett, Z.H., Shirtcliff, E.A., Dunn, E.C., Drury, S.S., May 2013. Neighborhood disorder and telomeres: connecting children’s exposure to community level stress and cellular response. Social Science and Medicine 85, 50e58. Theodosis, D.T., Bonhomme, R., Vitiello, S., Rougon, G., Poulain, D.A., 1999. Cell surface expression of polysialic acid on NCAM is a prerequisite for activity-dependent morphological neuronal and glial plasticity. Journal of Neuroscience 19, 10228e10236. Tochigi, M., Iwamoto, K., Bundo, M., Sasaki, T., Kato, N., Kato, T., February 2008. Gene expression profiling of major depression and suicide in the prefrontal cortex of postmortem brains. Neuroscience Research. 60 (2), 184e191. Tomasdottir, M.O., Sigurdsson, J.A., Petursson, H., et al., 2015. Self reported childhood difficulties, adult multimorbidity and allostatic load. A cross-sectional analysis of the Norwegian HUNT study. PLoS One 10 (6), e0130591. Tsankova, N.M., Berton, O., Renthal, W., Kumar, A., Neve, R.L., Nestler, E.J., 2006. Sustained hippocampal chromatin regulation in a mouse model of depression and antidepressant action. Nature Neuroscience 9, 519e525. van der Kooij, M.A., Fantin, M., Kraev, I., et al., April 2014. Impaired hippocampal neuroligin-2 function by chronic stress or synthetic peptide treatment is linked to social deficits and increased aggression. Neuropsychopharmacology 39 (5), 1148e1158. van der Kooij, M.A., Fantin, M., Rejmak, E., et al., 2014. Role for MMP-9 in stress-induced downregulation of nectin-3 in hippocampal CA1 and associated behavioural alterations. Nature Communications 5, 4995. Vollmann-Honsdorf, G.K., Flugge, G., Fuchs, E., 1997. Chronic psychosocial stress does not affect the number of pyramidal neurons in tree shrew hippocampus. Neuroscience Letters 233, 121e124. Vyas, A., Mitra, R., Rao, B.S.S., Chattarji, S., 2002. Chronic stress induces contrasting patterns of dendritic remodeling in hippocampal and amygdaloid neurons. Journal of Neuroscience 22, 6810e6818. Waddington, C.H., 1942. The epigenotype. Endeavour 1, 18e20. Wang, K., Xiang, X.H., He, F., et al., June 2010. Transcriptome profiling analysis reveals region-distinctive changes of gene expression in the CNS in response to different moderate restraint stress. Journal of Neurochemistry 113 (6), 1436e1446. Wood, G.E., Young, L.T., Reagan, L.P., McEwen, B.S., 2003. Acute and chronic restraint stress alter the incidence of social conflict in male rats. Hormones and Behavior 43, 205e213. Zohar, J., Yahalom, H., Kozlovsky, N., et al., November 2011. High dose hydrocortisone immediately after trauma may alter the trajectory of PTSD: interplay between clinical and animal studies. European Neuropsychopharmacology 21 (11), 796e809. Zoppi, S., Perez Nievas, B.G., Madrigal, J.L., Manzanares, J., Leza, J.C., Garcia-Bueno, B., March 2011. Regulatory role of cannabinoid receptor 1 in stress-induced excitotoxicity and neuroinflammation. Neuropsychopharmacology 36 (4), 805e818.

C H A P T E R

2

Cognitive and behavioral components of resilience to stress 1

Brian M. Iacoviello1, 2, Dennis S. Charney1

Department of Psychiatry, Icahn School of Medicine at Mount Sinai, New York, NY, United States; 2Discovery and Translational Research, Click Therapeutics, Inc., New York, NY, United States

Resilience: one of many possible responses to stress or trauma Stress and trauma can affect individuals in very different ways. On the one hand, studies have shown strong associations between a history of trauma exposure and the presence of psychiatric disorders. The most severe manifestations are often posttraumatic stress disorder (PTSD), depressive disorders, and substance use disorders. However, most times, stress or trauma exposure does not result in psychiatric disorders. A large study of a community sample estimated an approximately 9% risk of developing PTSD after trauma (Breslau et al., 1998). Clearly, some individuals exhibit a remarkable ability to endure great stress, torture, trauma, or disaster; we describe these individuals as “resilient.” Resilience refers to possessing a set of adaptive characteristics that enable an individual to cope with and recover from (or even thrive after) stress or trauma. Considering the range of stresses and traumatic experiences humans can face, the factors that contribute to resilience versus other outcomes including the emergence of psychiatric disorders are important to understand. Understanding these factors can help promote resilience in individuals before they even encounter stress or trauma and can inform the treatment of individuals struggling with stress or trauma. A set of psychosocial factors that appear to contribute to resilience after trauma exposure have been identified through anecdotal evidence from interviews with resilient individuals and research evidence from studies of trauma and disaster survivors. These factors comprise cognitive and behavioral components (see Table 2.1 for a description of the factors and their components), where cognitive components concern people’s patterns of thinking or core beliefs and behavioral components concern patterns of action or activity. In support of this conceptualization, a factor analysis of the items on a widely used tool used to assess resilience, the Connor-Davidson Resilience Scale (Connor and Davidson, 2003), suggests five

Stress Resilience https://doi.org/10.1016/B978-0-12-813983-7.00002-1

23

Copyright © 2020 Elsevier Inc. All rights reserved.

24

2. Cognitive and behavioral components of resilience to stress

TABLE 2.1

Cognitive and behavioral components of psychosocial factors associated with resilience. Components

Factor

Cognitive

Optimism

Maintain positive expectancies for the future.

Cognitive flexibility

Reappraise, reframe, and assimilate stress/trauma. Accept stress/trauma and failure as ingredients for growth.

Active coping skills (vs. passive)

Minimize continued appraisal of threat. Maintain positive self-regard.

Physical health

Behavioral

Actively seek help and resources; face your fears. Physical activity and exercise.

Social support network

Thinking one is “connected” versus alone.

Maintain a social support network. Connect with a resilient role model.

Personal moral compass

Adaptive, positive core beliefs; religious or spiritual beliefs; sense of purpose in life.

Altruistic behavior.

factors underlie resilience, which the authors described as: (1) tenacity and a sense of personal competence; (2) tolerance of negative affect and acceptance of the strengthening effects of stress; (3) acceptance of change and cultivating secure relationships; (4) sense of control; and (5) spiritual influences. Factors 1, 2, 3, and 4 include cognitive components: thinking patterns and core beliefs that lead one to believe they can endure. Factors 1 and 3 also include behavioral components: being active and engaged in one’s response to stress or traumatic situations and actively cultivating relationships and social support networks. Factor 5, spiritual influences, comprises both cognitive and behavioral componentsdfor example, maintaining spiritual or religious beliefs as a cognitive component and seeking out opportunities for altruistic behavior as a behavioral component. There are also neurobiological factors associated with resilience, including genetic factors, neurochemical systems involved in the stress response, and the functioning of specific neural networks (Charney, 2004; Feder et al., 2009) although these are beyond the scope of this chapter. Here we describe the psychosocial factors that contribute to resilience, organized into cognitive and behavioral components, along with recommendations for cultivating these factors.

Cognitive and behavioral components of the psychosocial factors associated with resilience Optimism Optimism comprises primarily cognitive elements. Optimism means maintaining positive expectancies for future events or outcomes (Carver et al., 2010). Optimism has typically been considered a personality dimension, suggesting it is likely more of a trait than a state characteristic. However, an individual’s degree of optimism can shift over time or across situations.

Cognitive and behavioral components of the psychosocial factors associated with resilience

25

Optimism has been associated with self-reported well-being among long-term breast cancer survivors (Carver et al., 2005), psychological adjustment during a life transition (Brissette et al., 2002), and reduced PTSD symptom severity after an earthquake (Ahmad et al., 2010). When encountering adversity, maintaining optimism for the future can buoy ones spirit and provide the stamina to endure, but optimism alone is not sufficient to foster resilience. James Stockdale, the highest-ranking naval officer held as a prisoner of war in Vietnam, was known for his resilience to this situation and provides insight regarding the role of optimism in resilience when he was asked “Who did not make it out of Vietnam?” Oh, that’s easy, the optimists. Oh, they were the ones who said, ‘We’re going to be out by Christmas.’ And Christmas would come, and Christmas would go. Then they’d say, ‘We’re going to be out by Easter.’ And Easter would come, and Easter would go. And then Thanksgiving, and then it would be Christmas again. And they died of a broken heart. Collins, (2014)

Cognitive flexibility Stockdale’s response illuminates another factor that is important for resilience: cognitive flexibility. Cognitive flexibility refers to the ability to reappraise one’s perception and experience, instead of being rigid in one’s perception. Reappraisal involves finding meaning and positivity in a situation, as well as acknowledging the negative or painful aspects. Reevaluating stressful or traumatic experiences can alter their perceived value or meaning. If one can learn to reframe thoughts about a traumatic event, assimilating these into their memories and beliefs about the event, one may be able to accept and eventually recover. Acceptance and assimilation of a traumatic experience into one’s life narrative involves acknowledging that experiences with stress or trauma can provide opportunities for growth, even when there is pain or distress. Optimism and cognitive flexibility can work in tandem; together they can enable an individual to maintain faith that they will endure while also accepting the harsh reality they face.

Active coping skills and a strong social support network Active coping skills involve both cognitive and behavioral components. Active rather than passive coping skills are often employed by resilient individualsdthey act to promote their own resilience. The cognitive component includes mindfulness for thoughts about situations and actively minimizing the appraisal of threat to avoid becoming consumed by fear. The behavioral component includes efforts to create positive statements about oneself, facing one’s fears instead of avoiding them, and efforts to seek the help and support of others. This is also related to another factor for promoting resilience, maintaining a strong social support network. Very few can “go it alone,” and resilient examples often acknowledge the invaluable social support around them. Close relationships can convey considerable emotional strength to an individual, and perceiving an available “safety net” can encourage acting in one’s own interest when confronting or recovering from stressful or traumatic situations. Recent studies of PTSD in returning Iraq/Afghanistan war veterans support this. One study found that PTSD was associated with greater relationship impairment, reduced social support, and impaired social functioning. Importantly, these social impairments were not simply consequences of PTSD. Reduced social support from the community

26

2. Cognitive and behavioral components of resilience to stress

and reduced availability of secure relationships appeared to mediate associations between PTSD and poor social functioning (Tsai et al., 2012). In another study, being in a relationship, having fewer psychosocial difficulties, and reporting greater perceptions of control and family support were all associated with resilience (Pietrzak and Southwick, 2011). Moreover, the presence of robust social support can influence one’s thinking about themselves and their worlds in a positive way. This can help protect against developing hopelessness and other negative psychological outcomes (Panzarella et al., 2006). Taken together, effective social support can engender strength to face fear and trauma and can minimize the experience of hopelessness while encouraging active coping.

Physical activity Physical activity is primarily a behavioral factor. Attending to one’s physical health can help promote resilience. Physical exercise improves physical hardiness and increases strength and stamina, which can increase the chances of survival in traumatic situations. Physical exercise results in positive effects on mood and self-esteem (Scully et al., 1998), as well as aspects of cognition and brain function (Hillman et al., 2008). Maintaining awareness of one’s physical hardiness during a traumatic situation can contribute to mental fortitude to endure. Improved mood and increased self-esteem resulting from physical exercise can also facilitate establishing and nurturing social relationships, which are important for promoting resilience.

A personal moral compass Embracing a personal moral compass involves both cognitive and behavioral factors. The cognitive component involves developing a set of values and holding a strong set of positive core beliefs about oneself and one’s role in their world. Studies have shown that hopelessness and depression can result when individuals maintain negative beliefs regarding the stability (persisting over time), globality (affecting different areas of one’s life), and internality (association with one’s own personal characteristics) of the negative life events that they encounter (Alloy et al., 1997). On the other hand, maintaining positive beliefs results in adaptive thinking, can help prevent the development of hopelessness, and encourages resilience. Observing one’s own behavior can also contribute to one’s beliefs about themselves and their worlds. Engaging in altruistic behavior toward others can result in positive self-beliefs and can be a factor that promotes resilience in the face of stress. Thus, altruism is an important behavioral component of embracing a personal moral compass, and it has been strongly associated with resilience in children and adults (Southwick et al., 2005; Leontopoulou, 2010). Altruistic behavior helps others but confers a sense of community and connectedness for the altruistic individual, which can also contribute to perceived meaning and purpose in life. In a study of primary care patients, purpose in life was a key factor associated with resilience and recovery from illness (Alim et al., 2008), and in a study of earthquake survivors, purpose in life was associated with reduced PTSD and depressive symptoms (Feder et al., 2013). For many, faith in conjunction with religion or spirituality is an important component of a personal moral compass. Religion or spirituality provide opportunities to gain understanding

Cultivating psychosocial factors to promote resilience

27

of questions about life and personal meaning. This can be particularly relevant in times of stress or trauma, when it can be hard to find positive meaning or value in the situation. However, doing so can aid in generating a healthy perspective of a traumatic situation and, accordingly, contribute to resilience. In fact, positive religious coping is associated with healthier physical and mental outcomes after surviving disaster (Smith et al., 2000) and in medically ill patients (Pargament et al., 2004). A large metaanalysis investigating the association between religious coping and psychological adjustment to stress found positive religious coping to have a moderate association with positive psychological adjustment (Ano and Vasconcelles, 2005).

Cultivating psychosocial factors to promote resilience Encourage optimism, attend to pessimism, and aspire for flexibility Optimistic attitudes can be hard to cultivate during times of stress or trauma. In those times, others can be relied upon to hold onto and convey optimism. Find someone who is able to express hope that the patient’s symptoms can improve or who can cite research and personal experience that things can get better. A positive attitude expressed by another can go a long way in encouraging some hope in the patient and can motivate them to continue to try. Unlike optimistic attitudes, negative or hopeless attitudes seem easy to develop during times of distress. Particularly, negative or hopeless attitudes should be addressed when encountered. Attend to common cognitive distortions (Burns, 1989) that underlie anxious and depressed thinking such as all or nothing thinking, overgeneralizing, disqualifying the positive, jumping to conclusions, catastrophizing, excessive “should” statements, and personalizing. Studies have also shown that hopelessness can stem from maintaining rigid and negative beliefs regarding the stability, globality, and internality of the life events that are encountered (Alloy et al., 1997), so maintaining relatively positive beliefs can result in more adaptive thinking, preventing the development of hopelessness and encouraging resilience. There are many anecdotal examples of the power of optimism and cognitive flexibility. As a general example, consider the history of runners attempting to “break the 4-minute mile,” which means running a full 1-mile in under 4 min. This is a story of what can happen when the “impossible” becomes seen as “possible” or what can happen with optimism and cognitive flexibility. Until 1954, runners, physiologists, and medical doctors believed that it was impossible for a human body to naturally achieve the speed necessary to run 1 mile in under 4 min (approximately 15 miles per hour). This belief was crystallized over many, many years in which no runner achieved the feat despite many trying. As long as the feat was believed to be impossible, there was no success at breaking through the 4-minute mile. Then, in the 1950s, some runners began to challenge the belief that it was humanely impossible to break a 4-minute mile. Without any evidence to support them, they decided to be cognitively flexible and optimistic and committed to the belief that it was possible for a human to run a 4-minute mile. They then set out to prove it. With the help of some fellow runners and teammates to pace him, on May 6, 1954, a British runner named Roger Bannister became the first human to break a 4-minute mile (he ran it in 3:59.4). Just the one example of a human breaking the 4-minute mile was all that was needed to challenge the belief that it was “impossible”; now

28

2. Cognitive and behavioral components of resilience to stress

this was something that was possible in the mind of runners. This optimistic shift in belief then enabled what was previously seen as impossible, and other runners miraculously began to accomplish the same feat. Just 2 months after Bannister first broke the 4-minute mile, he and an Australian runner named John Landy both ran 1 mile in under 4 minutes in the 1954 British Empire and Commonwealth Games. In the past 50 years, the mile record has been lowered numerous times, by approximately 17 s in total. As long as breaking the 4-minute mile was believed to be impossible, despite best efforts, it was not achieved; once it was believed to be possible, runners began to accomplish the feat. Cognitive flexibility and optimism facilitated a shift in belief that enabled the accomplishment for runners after Bannister.

Face your fears The initial, knee-jerk reaction to a fear- or anxiety-inducing situation is often to try as hard as we can to avoid it, to minimize the amount of fear or anxiety we experience. However, fear is a normal human experience, as it is intended to inform us about potential dangers in our environment. Listening to fear can help identify potential dangers, but it is also important to recognize that avoidance should not be the automatic reaction to fear. Some psychiatric disorders are characterized by nonacceptance of fear and maladaptive efforts to avoid fear, anxiety, or uncertainty (e.g., Hayes et al., 1996). Accepting fear and anxiety as part of the normal human experience, and pushing oneself to face fears instead of avoiding them, can help promote resilience. Stress inoculation, which involves deliberate prior exposure to manageable stressors, reduces the behavioral and physiological responses to later stressors (Meichenbaum, 1996). Prior exposure to stressors increases one’s sense of control and mastery of stressful situations and through desensitization reduces the amount of anxiety experienced when confronted with these situations, enabling one to learn to respond to stressors more adaptively. Facing one’s fears provides an opportunity for stress inoculation, learning to cope with fear actively and adaptively, and even strengthening self-esteem.

Connect with a resilient role model When searching for a resilient role model, seek someone who has survived adversity, disaster, or trauma. Or, resilient role models can be found in support groups or other groups of individuals who have encountered similar stresses or traumas. Modeling of behavior is a powerful method to learn new behavior(s), and so a resilient role model can help cultivate resilience-promoting characteristics via modeling and internalizing the experience of resilience. For example, role models who have successfully navigated a stressful life event might be able to teach cognitive flexibility by relaying their own experience of acceptance, reappraisal, and assimilation of traumatic experiences. Relaying their own unique story of cognitive flexibility can help normalize the difficulty and also provide hope for success. These role models can become a part of a social support network, providing opportunities to model other relevant behaviors including active coping skills and the search for purpose in life. Ideally a resilient role model will function as a coach would, focusing the learner on skills to practice and strengthen, using themselves and their own success as a real-life example to learn from.

Cultivating psychosocial factors to promote resilience

29

Form and maintain a supportive social network Since “few can go-it-alone,” having a social support network in place, on which one can rely during trauma or stress, can mean the difference between resilient outcomes versus the development of psychopathology. Social relations contribute to emotional strength, and social support can influence one’s beliefs about themselves and their worlds (Panzarella et al., 2006), which can facilitate optimism and positive self-regard. Supportive social networks also encourage active and adaptive coping behavior. For example, minimizing the appraisal of threat and acting in one’s own best interest are easier if a safety net is perceived in their social networks. These networks can involve friends, family, coworkers, spiritual advisors, mentors, role models, and others. Nurturing these relationships to establish support networks can be invaluable in promoting resilience.

Attend to physical health and well-being Establishing a physical exercise regimen and/or ensuring regular physical activity can have a number of benefits. Physical exercise contributes to physical hardiness and, practically speaking, the ability to survive in certain situations. Physical activity is known to contribute to improved mood and self-esteem (Scully et al., 1998) as well as aspects of cognition and brain function (Hillman et al., 2008). Physical exercise has not only physical health benefits but also mental health benefits. Physical activity prior to stress or trauma can increase self-esteem and optimism about the chances for survival. During stressful times, physical activity can improve mood and cognitive capacities for emotion regulation. Even after the stress or trauma has ended, physical exercise can still affect resilience, as physical activity can improve mood, emotion regulation, cognitive flexibility, etc., which can facilitate resilient outcomes. The key is to establish a physical exercise regimen with some regularity and to stick to it.

Attend to your personal moral compass; identify and foster your character strengths Attending to your personal moral compass involves developing and holding a set of core beliefs about yourself and your role in the world that are positive or adaptive. Being able to answer the question “who am I and what do I stand for?” with a set of beliefs about yourself is indicative of a strong personal moral compass. Being mindful for thoughts about yourself and your world is the first step in attending to your personal moral compass, and making sure that the beliefs that you hold are healthy and adaptive is key. This can involve challenging or reframing any core beliefs that are unhealthy, maladaptive, or generally negative, and practicing holding onto healthier, more adaptive ones. Spiritual or religious beliefs, including belief in a higher power, and spiritual or religious activities can also help guide your moral compass and should be pursued. As our own behaviors can also shape our beliefs about ourselves, engaging in behaviors that help others can influence our core beliefs in a positive direction. Seek opportunities to help others or better the world, or engage in behaviors that highlight your positive characteristics and strengths. We all have our relative strengths and weaknesses. Character strengths can include extroversion, emotion regulation capabilities, openness to new experience, and stress

30

2. Cognitive and behavioral components of resilience to stress

(or fear) tolerance and can be leveraged to cultivate the psychosocial factors that will promote resilience. For example, extroversion can help establish and nurture a supportive social network. Openness to new experiences can be relied on for stress inoculation and practicing facing one’s fears. The key is to learn to recognize our character strengths and engage them when confronting stressful situations. Identifying character strengths can also help identify relative weaknesses, which can then be honed and strengthened. Modifying a character strength or weakness typically involves training regularly and rigorously, as change requires systematic and disciplined activity. Commit to training in multiple areas and commit to a training regimen with enough frequency of practice to yield success.

References Ahmad, S., Feder, A., Lee, E.J., Wang, Y., Southwick, S.M., Schlackman, E., Bucholz, K., Alonso, A., Charney, D.S., 2010. Earthquake impact in a remote South Asian population: psychosocial factors and posttraumatic symptoms. Journal of Traumatic Stress 23, 408e412. Alim, T.N., Feder, A., Graves, R.E., Wang, Y., Weaver, J., Westphal, M., Alonso, A., Aigbogun, N.U., Smith, B.W., Doucette, J.T., Mellman, T.A., Lawson, W.B., Charney, D.S., 2008. Trauma, resilience and recovery in a highrisk African-American population. American Journal of Psychiatry 165, 1566e1575. Alloy, L.B., Just, N., Panzarella, C., 1997. Attributional style, daily life events, and hopelessness depression: subtype validation by prospective variability and specificity of symptoms. Cognitive Therapy and Research 21, 321e344. Ano, G.G., Vasconcelles, E.B., 2005. Religious coping and psychological adjustment to stress: a meta-analysis. Journal of Clinical Psychology 61, 461e480. Breslau, N., Kessler, R.C., Chilcoat, H.D., Schultz, L.R., Davis, G.C., Andreski, P., 1998. Trauma and posttraumatic stress disorder in the community: the 1996 detroit area survey of trauma. Archives of General Psychiatry 55, 626e632. Brissette, I., Scheier, M.F., Carver, C.S., 2002. The role of optimism and social network development, coping, and psychological adjustment during a life transition. Journal of Personality and Social Psychology 82, 102e111. Burns, D.D., 1989. The Feeling Good Handbook: Using the New Mood Therapy in Everyday Life. W. Morrow, New York. Carver, C.S., Scheier, M.F., Segerstrom, S.C., 2010. Optimism. Clinical Psychology Review 30, 879e889. Carver, C.S., Smith, R.G., Antoni, M.H., Petronis, V.M., Weiss, S., Derhagopian, R.P., 2005. Optimistic personality and psychosocial well-being during treatment predict psychosocial well-being among long-term survivors of breast cancer. Health Psychology 24, 508e516. Charney, D.S., 2004. Psychobiological mechanisms of resilience and vulnerability: implications for successful adaptation to extreme stress. American Journal of Psychiatry 161, 195e216. Collins, J (Producer)., 2014. The Stockdale Paradox. The Brutal Facts. (Audio podcast). Retrieved from: http://www. jimcollins.com/media_topics/brutal-facts.html. Connor, K.M., Davidson, J.R.T., 2003. Development of a new resilience scale: the Connor-Davidson resilience Scale (CD-RISC). Depression and Anxiety 18, 76e82. Feder, A., Ahmad, S., Lee, E.J., Morgan, J.E., Singh, R., Smith, B.W., Southwick, S.M., Charney, D.S., 2013. Coping and PTSD symptoms in Pakistani earthquake survivors: purpose in life, religious coping and social support. Journal of Affective Disorders 147, 156e163. Feder, A., Nestler, E., Charney, D.S., 2009. Psychobiology and molecular genetics of resilience. Nature Reviews Neuroscience 10, 446e457. Hayes, S.C., Wilson, K.G., Gifford, E.V., Follette, V.M., Strosahl, K., 1996. Experiential avoidance and behavioral disorders: a functional dimensional approach to diagnosis and treatment. Journal of Consulting and Clinical Psychology 64, 1152e1168. Hillman, C.H., Erickson, K.I., Kramer, A.F., 2008. Be smart, exercise your heart: exercise effects on brain and cognition. Nature Reviews Neuroscience 9, 58e65. Leontopoulou, S., 2010. An exploratory study of altruism in Greek children: relations with empathy, resilience and classroom climate. Psychology 1, 377e385.

References

31

Meichenbaum, D., 1996. Stress inoculation training for coping with stressors. Clinical Psychologist 49, 4e7. Panzarella, C., Alloy, L., Whitehouse, W., 2006. Expanded hopelessness theory of depression: on the mechanisms by which social support protects against depression. Cognitive Therapy and Research 30, 307e333. Pargament, K.I., Koenig, H.G., Tarakeshwar, N., Hahn, J., 2004. Religious coping methods as predictors of psychological, physical and spiritual outcomes among medically ill elderly patients: a two-year longitudinal study. Journal of Health Psychology 9, 713e730. Pietrzak, R.H., Southwick, S.M., 2011. Psychological resilience in OEF-OIF veterans: application of a novel classification approach and examination of demographic and psychosocial correlates. Journal of Affective Disorders 133, 560e568. Scully, D., Kremer, J., Meade, M.M., Graham, R., Dudgeon, K., 1998. Physical exercise and psychological well-being: a critical review. British Journal of Sports Medicine 32, 111e120. Smith, B.W., Pargament, K.I., Brant, C., Oliver, J.M., 2000. Noah revisited: religious coping and the impact of a flood. Journal of Community Psychology 28, 169e186. Southwick, S.M., Vythilingam, M., Charney, D.S., 2005. The psychobiology of depression and resilience to stress: implications for prevention and treatment. Annual Review of Clinical Psychology 1, 255e291. Tsai, J., Harpaz-Rotem, I., Pietrzak, R.H., Southwick, S.M., 2012. The role of coping, resilience, and social support in mediating the relation between PTSD and social functioning in veterans returning from Iraq and Afghanistan. Psychiatry 75, 135e149.

C H A P T E R

3

Resilience as a process instead of a trait David M. Lyons, Alan F. Schatzberg Department of Psychiatry and Behavioral Sciences, Stanford University, Stanford, CA, United States

Introduction Stress resilience is crucial for health and well-being, but commonly accepted conceptual frameworks for resilience research have not yet emerged (Aburn et al., 2016; Johnston et al., 2015). Trait perspectives suggest that cognitive capabilities, personality characteristics, and neurobiological factors work along with environmental assets to make certain individuals resilient (Feder et al., 2009; Johnson, Panagioti, Bass, Ramsey and Harrison, 2017; Kalisch et al., 2015; Ungar, 2015). Process perspectives instead suggest that coping with stress builds resilience through learning and memory mechanisms (DiCorcia and Tronick, 2011; Lee et al., 2016a; Lyons et al., 2009). Experiences that are emotionally stressful but not traumatic promote coping and stress resilience by tagging learned information for memory consolidation (Bergado et al., 2011; Dunsmoor et al., 2015; Sotgiu and Mormont, 2008). Emotional experiences are known to enhance learning and memory mechanisms (Cahill and McGaugh, 1998; LaBar and Cabeza, 2006; Lisman et al., 2011). Emotional learning and memory are adaptive (Cosmides and Tooby, 2013) and often utilized in therapeutic settings to foster recovery and resilience (Lane et al., 2015). Variously described in studies of humans as inoculating, steeling, or toughening (Dienstbier, 1989; Garmezy et al., 1984; Russo et al., 2012; Rutter, 2013), the notion that learning to cope with stress builds resilience is supported by nonhuman primate research. In natural and seminatural conditions, squirrel monkey mothers and other group members periodically leave newly weaned offspring beginning at 3e6 months of age to forage for food on their own (Lyons et al., 1998). At this stage of development, offspring are approximately half their adult body size. Initially, brief mother-infant separations studied in controlled experimental conditions elicit distress peep calls and increase plasma levels of the stress hormone cortisol with partial habituation of these measures observed over

Stress Resilience https://doi.org/10.1016/B978-0-12-813983-7.00003-3

33

Copyright © 2020 Elsevier Inc. All rights reserved.

34

3. Learning to cope builds resilience

repeated separations (Coe et al., 1983; Hennessy, 1986). Later in life, monkeys exposed to intermittent separations show fewer behavioral indications of anxiety, diminished stress levels of cortisol, and enhanced glucocorticoid feedback regulation of the hypothalamicpituitary-adrenal (HPA) axis compared with monkeys not exposed to intermittent separations (Levine and Mody, 2003; Lyons et al., 1999; Lyons, Parker and Schatzberg, 2010b; Parker et al., 2004; Parker et al., 2006). Similar examples have been reported for human children learning to cope with everyday stress (DiCorcia and Tronick, 2011), separation stress (Poulton et al., 2001), family stress (Hagan et al., 2014), work-related stress (Mortimer and Staff, 2004), and other diverse stressful life events (Boyce and Chesterman, 1990). Learning to cope is not, of course, limited to sensitive or critical periods in primate postnatal development. Learning to cope with intermittent stress increases adult monkey hippocampal neurogenesis and enhances the expression of genes involved in cell proliferation and survival (Lyons et al., 2010a). Learning to cope also protects adult monkeys against subsequent stress-induced deficits in behavior on tests of emotionality and diminishes the HPA axis neuroendocrine stress response (Lee et al., 2014). Adult humans who survive earthquakes or floods subsequently respond to natural disasters with diminished anxiety (Norris and Murrell, 1988) and less depressed affect (Knight et al., 2000) compared with inexperienced survivors. Since learning can never be directly observed (Staddon, 2016), learning to cope is unavoidably inferred from behavior change. Here we describe studies of learning to cope and resilience in mice. Instead of screening for the presence of traits that occur in resilient individuals or the absence of vulnerability to stress (Feder et al., 2009; Russo et al., 2012), we focus on learning to cope with stress and the process of building resilience. Considering resilience as a process implies that it can be modified and improved in people with or without preexisting psychopathologies (Waugh and Koster, 2015). Humans may more closely resemble various nonhuman primates, but the availability of research tools for dissecting causal pathways that link behavior and brain is much greater in mice compared with monkeys (Gerits and Vanduffel, 2013; Huang and Zeng, 2013). Therefore, mouse models offer essential opportunities to bridge the gap between basic and applied resilience research.

Learning-to-cope training Learning-to-cope sessions of training were designed for mice on the basis of evidence that mild but not minimal nor severe stress exposure provides opportunities to learn, practice, and improve coping as described by U-shaped functions (Russo et al., 2012; Sapolsky, 2015; Seery et al., 2010). In addition to the qualities or intensities of stress exposure, temporal aspects differentially contribute to vulnerability versus resilience. Chronic severe stress leads to vulnerability (Brosschot, 2010; Charney and Manji, 2004; Duman, 2009), whereas intermittent stress exposures interspersed with undisturbed periods of recovery provide repeated opportunities to learn, practice, and improve coping with subsequent gains in stress resilience (DiCorcia and Tronick, 2011; Lyons et al., 2010b). For studies of learning to cope in mice, we modified a protocol commonly used to investigate severe social stress. Instead of direct or continuous exposure to an aggressive, samesex, social stranger (Golden et al., 2011), learning-to-cope sessions of training are conducted

Learning to cope inferred from hormones and behavior

35

every other day for 15 min behind a mesh-screen barrier in the cage of a reproductively experienced, same-sex, resident stranger (Brockhurst et al., 2015). Immediately after each training session, subjects are returned to their home cage to allow ample time for recovery and consolidation of memory in familiar undisturbed conditions. Resident strangers are housed individually to promote defense behavior, and their prior reproductive experiences enhance same-sex aggression (Ferrari et al., 1996; Miczek et al., 2001; Palanza et al., 1996). Male resident mice attack same-sex intruders more often than do female residents, but females engage one another in similar levels of agonistic behavior compared with same-sex interactions of male mice (Clipperton-Allen et al., 2011). Agonistic behavior of female mice includes acts of aggression aimed at establishing social dominance instead of attacks to evict same-sex intruders. Attacks are salient but not necessary for learning-to-cope training because we employ a mesh-screen barrier to prevent all attacks as well as all physical injuries while allowing noncontact social interaction.

Learning to cope inferred from hormones and behavior Learning-to-cope sessions of training in mice subsequently diminish immobility as a measure of behavioral despair on tail-suspension tests (Brockhurst et al., 2015). Tail suspension elicits neuroendocrine stress responses (S. B. Johnson et al., 2016), and diminished immobility is thought to reflect active coping with tail-suspension stress. Learning-to-cope training also diminishes subsequent freezing in the open field as a measure of anxiety-like behavior and decreases exploration latencies for novel and familiar objects (Brockhurst et al., 2015). Less freezing, shorter latencies, and diminished immobility do not reflect nonspecific activity insofar as learning-to-cope training does not increase locomotor activity (Brockhurst et al., 2015). Greater effect sizes are observed for novel compared with familiar object-exploration latencies, but object recognition memory is not required to show learning-to-cope training effects on this behavioral test. Learning-to-cope main effects in mice are significant across novel and familiar object-exploration latencies (Brockhurst et al., 2015). Shorter latencies reflect enhanced curiosity, and curiosity has been associated with stress coping and resilience in humans (Denneson et al., 2017) and nonhuman primates (Parker et al., 2007). Initially, we reported learning-to-cope training effects for male mice (Fig. 3.1) and recently confirmed nearly identical effects in female mice (Lyons et al., 2018). Comparable findings in male and female monkeys (Lee et al., 2014, 2016b; Lyons et al., 2010a) support the hypothesis that learning-to-cope training is mediated by conserved neural mechanisms. Convergent results in monkeys and mice also minimize false-positive findings and enhance translational relevance (Ciesielski et al., 2014). Translation has commanded considerable attention because of recent uncertainties about generalization across different species given inevitable biological variation, the use of diverse experimental manipulations, and various ways to operationalize complex outcomes of interest (Institute of Medicine, 2013). Reproducibility across distinct models that utilize different species and both sexes helps to ensure that observed outcomes generalize to broader contexts. The hypothesis that learning to cope with stress builds resilience is further supported by human psychotherapies. Intermittent exposure to mildly stressful situations is a feature of stress inoculation training for people who work in conditions where performance in the

36

3. Learning to cope builds resilience

FIGURE 3.1 Learning to cope inferred from behavior. Learning-to-cope training subsequently decreases (A) immobility on tail-suspension tests, (B) freezing in the open field, and (C) exploration latencies for novel (Nov) and familiar (Fam) objects (mean
SEM, n ¼ 12 mice per treatment, *P < .05). Adapted with permission from “Stress inoculation modeled in mice”, by Brockhurst, J., Cheleuitte-Nieves, C., Buckmaster, C.L., Schatzberg, A.F., Lyons, D.M., 2015. Stress inoculation modeled in mice. Translational Psychiatry 5, e537. Copyright 2015 by Nature Publishing Group.

face of adversity is required, for example, medical and military personnel, police, firefighters, and rescue workers (Meichenbaum and Novaco, 1985; Saunders et al., 1996; Stetz et al., 2007). Exposure psychotherapies likewise train people to imagine a graded series of stress-inducing situations and encourage interaction with stressors in vivo (McNally, 2007). These procedures promote learning (Craske et al., 2008) and provide opportunities to practice acquired coping skills (Serino et al., 2014). Animal models of exposure psychotherapies often focus on learned extinction of conditioned fear (Milad and Quirk, 2012). Extinction occurs when a conditioned stimulus (CS) that was previously paired with an unconditioned stimulus (US) is repeatedly presented on its own. Repeated presentation of the CS alone results in new learning and subsequent inhibition of the conditioned fear response (Milad and Quirk, 2012). Far less researched, but of equal importance, are indications that repeated presentation of the US alone also inhibits conditioned fear responses by devaluing or reducing the impact of the US through a process called US habituation (Rauhut et al., 2001; Storsve et al., 2010). Inhibitory effects of CS extinction do not generalize to contexts that differ from those in which CS extinction learning occurred (Bouton, 2002; Rauhut et al., 2001). Context specificity limits the utility of animal models for exposure psychotherapies based on CS extinction (Craske et al., 2008; McNally, 2007). In contrast, certain models of US habituation are minimally responsive to modulation by contextual cues (Churchill et al., 1987; Evans and Hammond, 1983; Grissom et al., 2007; Hall and Honey, 1989; Jordan et al., 2000; Nyhuis et al., 2010; Rauhut et al., 2001). In this regard, US habituation resembles aspects of learning to cope. Learning-to-cope training effects generalize in mice to various test contexts as exemplified by diminished immobility during tail suspension, less freezing in the open field, and

Neurobiology of learning to cope

37

decreased object-exploration latencies (Fig. 3.1). Glucocorticoid stress hormone responses to repeated restraint are also diminished by learning-to-cope training compared with no-training controls (Fig. 3.2). Learning to cope is stressful, however, insofar as repeated training sessions increase glucocorticoids without habituation (Fig. 3.3). Learning to cope does not therefore represent habituation as a form of nonassociative learning and appears to reflect associative learning and memory inferred from behavior change.

Neurobiology of learning to cope In an unbiased search for neural markers of learning to cope, we discovered increased stargazin (also called TARP gamma-2 or CACNG2) expression in anterior cingulate cortex of monkeys (Lee et al., 2016a). The anterior cingulate cortex is involved in learning, remote memory, cognitive control, emotion, and HPA axis regulation (Etkin et al., 2011; Herman, 2013; Ochsner et al., 2012; Wang et al., 2012; Weible, 2013). Stargazin regulates AMPA receptor trafficking (Chen et al., 2000; Jackson and Nicoll, 2011; Vandenberghe et al., 2005) by interacting with scaffold proteins of the postsynaptic density (Bats et al., 2007). AMPA receptor trafficking plays a key role in synaptic plasticity as a mechanism for learning (Huganir and Nicoll, 2013) viewed functionally in terms of behavior change. Eyeblink conditioning, for example, increases stargazin in male rat cerebellum (Kim and Thompson, 2011), and we found that learning-to-cope training increases stargazin in the anterior cingulate cortex of monkeys (Lee et al., 2016a). Results from monkeys were therefore tested for reproducibility in mice. Brain tissues were collected from mice that were or were not behaviorally tested for emotionality after learning-to-cope training or the no-training control (Lee et al., 2016a).

FIGURE 3.2 Learning to cope diminishes subsequent stress hormone responses to repeated restraint. Tail vein plasma corticosterone levels in undisturbed home cage baseline conditions (Base) and immediately after the first, third, and seventh restraint-stress test sessions (mean
SEM, n ¼ 8 mice per treatment, *P < .01). Adapted with permission from “Stress inoculation modeled in mice”, by Brockhurst, J., Cheleuitte-Nieves, C., Buckmaster, C.L., Schatzberg, A.F., Lyons, D.M., 2015. Stress inoculation modeled in mice. Translational Psychiatry 5, e537. Copyright 2015 by Nature Publishing Group.

38

3. Learning to cope builds resilience

FIGURE 3.3 Learning to cope is stressful. Tail vein plasma corticosterone levels in undisturbed home cage baseline conditions (Base) and immediately after the 1st, 3rd, 7th, and 11th learning-to-cope training sessions (mean
SEM, n ¼ 8 mice, *P < .01 Fishers protected t-tests relative to Base). Adapted with permission from “Stress inoculation modeled in mice”, by Brockhurst, J., Cheleuitte-Nieves, C., Buckmaster, C.L., Schatzberg, A.F., Lyons, D.M., 2015. Stress inoculation modeled in mice. Translational Psychiatry 5, e537. Copyright 2015 by Nature Publishing Group.

Training main effects for anterior cingulate cortex stargazin in mice were discerned, but neither the behavioral testing main effect nor behavioral testing-by-training interaction was significant (Fig. 3.4). Results indicate that increased stargazin is caused by learning-to-cope training rather than subsequent behavioral testing. Increased stargazin appears to be selective for the anterior cingulate cortex insofar as no change in stargazin was observed in hippocampus or amygdala (Lee et al., 2016a).

FIGURE 3.4 Increased stargazin is caused by learning-to-cope training rather than subsequent behavioral testing. Anterior cingulate cortex stargazin in mice randomized to learning-to-cope training versus the no-training control. Mice from each training treatment were exposed or not to subsequent behavioral testing for emotionality (N ¼ 11e12 mice; mean
SEM; *P ¼ .016). Adapted with permission from “Learning to cope with stress modulates anterior cingulate cortex stargazin expression in monkeys and mice”, by Lee, A.G., Capanzana, R., Brockhurst, J., Cheng, M. Y., Buckmaster, C.L., Absher, D., Schatzberg, A.F., Lyons, D.M., 2016a. Learning to cope with stress modulates anterior cingulate cortex stargazin expression in monkeys and mice. Neurobiology of Learning and Memory 131, 95e100. Copyright 2016 by Elsevier.

Neurobiology of learning to cope

39

As evidence of learning-to-cope training effects distinct from stress exposures per se, increased stargazin in the anterior cingulate cortex correlates inversely with diminished emotionality in mice (Lee et al., 2016a). Specifically, increased stargazin in the anterior cingulate cortex correlates with diminished object-exploration latencies and with diminished immobility on tail-suspension tests. Increased stargazin did not correlate with freezing in the open field, and stargazin was not measured in mice monitored for corticosterone in response to repeated restraint stress. Array data from the anterior cingulate cortex of monkeys also indicate that increased stargazin induced by learning-to-cope training correlates with increased AMPA receptor subunit GluA1 (Fig. 3.5). Synaptic delivery of GluA1 is enhanced by stargazin in vitro (Chen et al., 2003; Tomita et al., 2007) and is a well-known correlate of learning and memory in vivo (Kessels and Malinow, 2009; Mitsushima et al., 2011; Rumpel et al., 2005). AMPA receptor neurotransmission in the anterior cingulate cortex is required for learning in rats (Wang et al., 2012), and GluA1 is increased in mice that learned to cope with stress inferred from diminished neuroendocrine measures (Schmidt et al., 2010). Diminished neuroendocrine stress responses in monkeys that we previously described elsewhere (Lyons et al., 2007) were reanalyzed and found to correlate with increased GluA1 in the anterior cingulate cortex (Fig. 3.5). Synaptic AMPA receptor trafficking as a correlate of learning is modulated by dopamine acting in concert with other signaling molecules (Ouyang et al., 2017; Tritsch and Sabatini, 2012; Wolf, 2010). Dopaminergic projections to corticolimbic regions are known to be activated by aversive or stressful events (Holly and Miczek, 2016). Therefore, we hypothesize that learning-to-cope training activates dopaminergic projections to enhance AMPA receptor trafficking in the anterior cingulate cortex as a mechanism of synaptic plasticity for learning to facilitate the process of building stress resilience.

FIGURE 3.5 Increased anterior cingulate cortex GluA1 in monkeys correlates with (A) increased stargazin (r ¼ 0.85, P ¼ .0004) and with (B) diminished adrenocorticotropic hormone (ACTH) measured immediately after restraint stress (r ¼ 0.71, P ¼ .009). Monkeys trained to cope (circles) express higher levels of GluA1 (P ¼ .04) and stargazin (P ¼ .03) compared with no-training controls (squares). Diminished ACTH responses to restraint stress in monkeys trained to cope are nearly significantly different from controls (P ¼ .11).

40

3. Learning to cope builds resilience

Limitations Our findings should be interpreted along with potential limitations. Both learning-to-cope training and subsequent testing were conducted after onset of lights-on, and this schedule may have altered circadian aspects of emotionality. Training and testing also both required transfer of mice from their home cage into new environments. Studies of repeated cage transfers alone are needed, but contextual differences between training and testing generally enhance behavioral and neuroendocrine measures of arousal (Bouton et al., 2006; Herman, 2013) instead of producing observed reductions in emotionality. More broadly, the particular experiences that promote learning-to-cope training and resilience in mice are unknown, but experiential details gleaned from mice will probably not translate to humans. Therefore, we focus on neurobiology instead of dissecting experiential details. Lastly, the pursuit of convergent evidence in mice and monkeys may enhance translational relevance (Ciesielski et al., 2014), but this approach also increases the risk of falsely disregarding species differences as negative or unimportant results.

Conclusions Our findings suggest a new framework for stress resilience research. In addition to investigating how severe stress damages behavior and brain (Charney and Manji, 2004; Duman, 2009), we propose a complementary approach focused on learning to cope with stress and the process of building resilience. Mechanisms of learning to cope identified in animal models may provide novel targets for new treatments of stress disorders in humans. Pharmacological facilitation of coping shifts attention from neuropathology to consider neural mechanisms that mediate coping as targets for building stress resilience. Although not fully complete, this framework supersedes narrower views that regard stress as solely destructive and overlook its broader role in behavior, neurobiology, and human mental health.

References Aburn, G., Gott, M., Hoare, K., 2016. What is resilience? an integrative review of the empirical literature. Journal of Advanced Nursing 72, 980e1000. Bats, C., Groc, L., Choquet, D., 2007. The interaction between stargazin and PSD-95 regulates AMPA receptor surface trafficking. Neuron 53, 719e734. Bergado, J.A., Lucas, M., Richter-Levin, G., 2011. Emotional taggingea simple hypothesis in a complex reality. Progress in Neurobiology 94, 64e76. Bouton, M.E., 2002. Context, ambiguity, and unlearning: sources of relapse after behavioral extinction. Biological Psychiatry 52, 976e986. Bouton, M.E., Westbrook, R.F., Corcoran, K.A., Maren, S., 2006. Contextual and temporal modulation of extinction: behavioral and biological mechanisms. Biological Psychiatry 60, 352e360. Boyce, W.T., Chesterman, E., 1990. Life events, social support, and cardiovascular reactivity in adolescence. Journal of Developmental and Behavioral Pediatrics 11, 105e111. Brockhurst, J., Cheleuitte-Nieves, C., Buckmaster, C.L., Schatzberg, A.F., Lyons, D.M., 2015. Stress inoculation modeled in mice. Translational Psychiatry 5, e537. Brosschot, J.F., 2010. Markers of chronic stress: prolonged physiological activation and (un)conscious perseverative cognition. Neuroscience and Biobehavioral Reviews 35, 46e50.

References

41

Cahill, L., McGaugh, J.L., 1998. Mechanisms of emotional arousal and lasting declarative memory. Trends in Neurosciences 21, 294e299. Charney, D.S., Manji, H.K., 2004. Life stress, genes, and depression: multiple pathways lead to increased risk and new opportunities for intervention. Science’s STKE 2004 (225), re5. Chen, L., Chetkovich, D.M., Petralia, R.S., Sweeney, N.T., Kawasaki, Y., Wenthold, R.J., et al., 2000. Stargazin regulates synaptic targeting of AMPA receptors by two distinct mechanisms. Nature 408, 936e943. Chen, L., El-Husseini, A., Tomita, S., Bredt, D.S., Nicoll, R.A., 2003. Stargazin differentially controls the trafficking of alpha-amino-3-hydroxyl-5-methyl-4-isoxazolepropionate and kainate receptors. Molecular Pharmacology 64, 703e706. Churchill, M., Remington, B., Siddle, D.A., 1987. The effects of context change on long-term habituation of the orienting response in humans. Quarterly Journal of Experimental Psychology B 39, 315e338. Ciesielski, T.H., Pendergrass, S.A., White, M.J., Kodaman, N., Sobota, R.S., Huang, M., Williams, S.M., 2014. Diverse convergent evidence in the genetic analysis of complex disease: coordinating omic, informatic, and experimental evidence to better identify and validate risk factors. BioData Mining 7, 10. Clipperton-Allen, A.E., Almey, A., Melichercik, A., Allen, C.P., Choleris, E., 2011. Effects of an estrogen receptor alpha agonist on agonistic behaviour in intact and gonadectomized male and female mice. Psychoneuroendocrinology 36, 981e995. Coe, C.L., Glass, J.C., Wiener, S.G., Levine, S., 1983. Behavioral, but not physiological, adaptation to repeated separation in mother and infant primates. Psychoneuroendocrinology 8, 401e409. Cosmides, L., Tooby, J., 2013. Evolutionary psychology: new perspectives on cognition and motivation. Annual Review of Psychology 64, 201e229. Craske, M.G., Kircanski, K., Zelikowsky, M., Mystkowski, J., Chowdhury, N., Baker, A., 2008. Optimizing inhibitory learning during exposure therapy. Behaviour Research and Therapy 46, 5e27. Denneson, L.M., Smolenski, D.J., Bush, N.E., Dobscha, S.K., 2017. Curiosity improves coping efficacy and reduces suicidal ideation severity among military veterans at risk for suicide. Psychiatry Research 249, 125e131. DiCorcia, J.A., Tronick, E., 2011. Quotidian resilience: exploring mechanisms that drive resilience from a perspective of everyday stress and coping. Neuroscience and Biobehavioral Reviews 35, 1593e1602. Dienstbier, R.A., 1989. Arousal and physiological toughness: implications for mental and physical health. Psychological Review 96, 84e100. Duman, R.S., 2009. Neuronal damage and protection in the pathophysiology and treatment of psychiatric illness: stress and depression. Dialogues in Clinical Neuroscience 11, 239e255. Dunsmoor, J.E., Murty, V.P., Davachi, L., Phelps, E.A., 2015. Emotional learning selectively and retroactively strengthens memories for related events. Nature 520, 345e348. Etkin, A., Egner, T., Kalisch, R., 2011. Emotional processing in anterior cingulate and medial prefrontal cortex. Trends in Cognitive Sciences 15, 85e93. Evans, J.G., Hammond, G.R., 1983. Differential generalization of habituation across contexts as a function of stimulus significance. Nov 1983. Animal Learning and Behavior 11, 431e434. Feder, A., Nestler, E.J., Charney, D.S., 2009. Psychobiology and molecular genetics of resilience. Nature Reviews Neuroscience 10, 446e457. Ferrari, P.F., Palanza, P., Rodgers, R.J., Mainardi, M., Parmigiani, S., 1996. Comparing different forms of male and female aggression in wild and laboratory mice: an ethopharmacological study. Physiology and Behavior 60, 549e553. Garmezy, N., Masten, A.S., Tellegen, A., 1984. The study of stress and competence in children: a building block for developmental psychopathology. Child Development 55, 97e111. Gerits, A., Vanduffel, W., 2013. Optogenetics in primates: a shining future? Trends in Genetics 29, 403e411. Golden, S.A., Covington 3rd, H.E., Berton, O., Russo, S.J., 2011. A standardized protocol for repeated social defeat stress in mice. Nature Protocols 6, 1183e1191. Grissom, N., Iyer, V., Vining, C., Bhatnagar, S., 2007. The physical context of previous stress exposure modifies hypothalamic-pituitary-adrenal responses to a subsequent homotypic stress. Hormones and Behavior 51, 95e103. Hagan, M.J., Roubinov, D.S., Purdom Marreiro, C.L., Luecken, L.J., 2014. Childhood interparental conflict and HPA axis activity in young adulthood: examining nonlinear relations. Developmental Psychobiology 56, 871e880.

42

3. Learning to cope builds resilience

Hall, G., Honey, R.C., 1989. Contextual effects in conditioning, latent inhibition, and habituation: associative and retrieval functions of contextual cues. Journal of Experimental Psychology: Animal Behavior Processes 15, 232e241. Hennessy, M.B., 1986. Multiple, brief maternal separations in the squirrel monkey: changes in hormonal and behavioral responsiveness. Physiology and Behavior 36, 245e250. Herman, J.P., 2013. Neural control of chronic stress adaptation. Frontiers in Behavioral Neuroscience 7, 61. Holly, E.N., Miczek, K.A., 2016. Ventral tegmental area dopamine revisited: effects of acute and repeated stress. Psychopharmacology 233, 163e186. Huang, Z.J., Zeng, H., 2013. Genetic approaches to neural circuits in the mouse. Annual Review of Neuroscience 36, 183e215. Huganir, R.L., Nicoll, R.A., 2013. AMPARs and synaptic plasticity: the last 25 years. Neuron 80, 704e717. Institute of Medicine, IOM, 2013. Improving the Utility and Translation of Animal Models for Nervous System Disorders: Workshop Summary. The National Academies Press, Washington, D.C. Jackson, A.C., Nicoll, R.A., 2011. Stargazin (TARP gamma-2) is required for compartment-specific AMPA receptor trafficking and synaptic plasticity in cerebellar stellate cells. Journal of Neuroscience 31, 3939e3952. Johnson, J., Panagioti, M., Bass, J., Ramsey, L., Harrison, R., 2017. Resilience to emotional distress in response to failure, error or mistakes: a systematic review. Clinical Psychology Review 52, 19e42. Johnson, S.B., Emmons, E.B., Anderson, R.M., Glanz, R.M., Romig-Martin, S.A., Narayanan, N.S., LaLumiere, R.T., Radley, J.J., 2016. A basal forebrain site coordinates the modulation of endocrine and behavioral stress responses via divergent neural pathways. Journal of Neuroscience 36, 8687e8699. Johnston, M.C., Porteous, T., Crilly, M.A., Burton, C.D., Elliott, A., Iversen, L., McArdle, K., Murray, A., Phillips, L.H., Black, C., 2015. Physical disease and resilient outcomes: a systematic review of resilience definitions and study methods. Psychosomatics 56, 168e180. Jordan, W.P., Strasser, H.C., McHale, L., 2000. Contextual control of long-term habituation in rats. Journal of Experimental Psychology: Animal Behavior Processes 26, 323e339. Kalisch, R., Muller, M.B., Tuscher, O., 2015. A conceptual framework for the neurobiological study of resilience. Behavioral and Brain Sciences 38, e92. Kessels, H.W., Malinow, R., 2009. Synaptic AMPA receptor plasticity and behavior. Neuron 61, 340e350. Kim, S., Thompson, R.F., 2011. c-Fos, arc, and stargazin expression in rat eyeblink conditioning. Behavioral Neuroscience 125, 117e123. Knight, B.G., Gatz, M., Heller, K., Bengtson, V.L., 2000. Age and emotional response to the Northridge earthquake: a longitudinal analysis. Psychology and Aging 15, 627e634. LaBar, K.S., Cabeza, R., 2006. Cognitive neuroscience of emotional memory. Nature Reviews Neuroscience 7, 54e64. Lane, R.D., Ryan, L., Nadel, L., Greenberg, L., 2015. Memory reconsolidation, emotional arousal, and the process of change in psychotherapy: new insights from brain science. Behavioral and Brain Sciences 38, e1. Lee, A.G., Buckmaster, C.L., Yi, E., Schatzberg, A.F., Lyons, D.M., 2014. Coping and glucocorticoid receptor regulation by stress inoculation. Psychoneuroendocrinology 49, 272e279. Lee, A.G., Capanzana, R., Brockhurst, J., Cheng, M.Y., Buckmaster, C.L., Absher, D., Scahtzber, A.F., Lyons, D.M., 2016a. Learning to cope with stress modulates anterior cingulate cortex stargazin expression in monkeys and mice. Neurobiology of Learning and Memory 131, 95e100. Lee, A.G., Nechvatal, J.M., Shen, B., Buckmaster, C.L., Levy, M.J., Chin, F.T., Schatzberg, A.F., Lyons, D.M., 2016b. Striatial dopamine D2/3 receptor regulation by stress inoculation in squirrel monkeys. Neurobiol Stress 3, 68e73. Levine, S., Mody, T., 2003. The long-term psychobiological consequences of intermittent postnatal separation in the squirrel monkey. Neuroscience and Biobehavioral Reviews 27, 83e89. Lisman, J., Grace, A.A., Duzel, E., 2011. A neoHebbian framework for episodic memory; role of dopamine-dependent late LTP. Trends in Neurosciences 34, 536e547. Lyons, D.M., Buckmaster, P.S., Lee, A.G., Wu, C., Mitra, R., Duffey, L.M., Buckmaster, C.L., Her, S., Patel, P.D., Schatzberg, A.F., 2010a. Stress coping stimulates hippocampal neurogenesis in adult monkeys. Proceedings of the National Academy of Sciences of the United States of America 107, 14823e14827. Lyons, D.M., Buckmaster, C.L., Schatzberg, A.F., et al., 2018. Learning to actively cope with stress in female mice. Psychoneuroendocrinology 96, 78e83. Lyons, D.M., Kim, S., Schatzberg, A.F., Levine, S., 1998. Postnatal foraging demands alter adrenocortical activity and psychosocial development. Developmental Psychobiology 32, 285e291.

References

43

Lyons, D.M., Martel, F.L., Levine, S., Risch, N.J., Schatzberg, A.F., 1999. Postnatal experiences and genetic effects on squirrel monkey social affinities and emotional distress. Hormones and Behavior 36, 266e275. Lyons, D.M., Parker, K.J., Katz, M., Schatzberg, A.F., 2009. Developmental cascades linking stress inoculation, arousal regulation, and resilience. Frontiers in Behavioral Neuroscience 3, 32. Lyons, D.M., Parker, K.J., Schatzberg, A.F., 2010b. Animal models of early life stress: implications for understanding resilience. Developmental Psychobiology 52, 402e410. Lyons, D.M., Parker, K.J., Zeitzer, J.M., Buckmaster, C.L., Schatzberg, A.F., 2007. Preliminary evidence that hippocampal volumes in monkeys predict stress levels of adrenocorticotropic hormone. Biological Psychiatry 62, 1171e1174. McNally, R.J., 2007. Mechanisms of exposure therapy: how neuroscience can improve psychological treatments for anxiety disorders. Clinical Psychology Review 27, 750e759. Meichenbaum, D., Novaco, R., 1985. Stress inoculation: a preventative approach. Issues in Mental Health Nursing 7, 419e435. Miczek, K.A., Maxson, S.C., Fish, E.W., Faccidomo, S., 2001. Aggressive behavioral phenotypes in mice. Behavioural Brain Research 125, 167e181. Milad, M.R., Quirk, G.J., 2012. Fear extinction as a model for translational neuroscience: ten years of progress. Annual Review of Psychology 63, 129e151. Mitsushima, D., Ishihara, K., Sano, A., Kessels, H.W., Takahashi, T., 2011. Contextual learning requires synaptic AMPA receptor delivery in the hippocampus. Proceedings of the National Academy of Sciences of the United States of America 108, 12503e12508. Mortimer, J.T., Staff, J., 2004. Early work as a source of developmental discontinuity during the transition to adulthood. Development and Psychopathology 16, 1047e1070. Norris, F.H., Murrell, S.A., 1988. Prior experience as a moderator of disaster impact on anxiety symptoms in older adults. American Journal of Community Psychology 16, 665e683. Nyhuis, T.J., Sasse, S.K., Masini, C.V., Day, H.E., Campeau, S., 2010. Lack of contextual modulation of habituated neuroendocrine responses to repeated audiogenic stress. Behavioral Neuroscience 124, 810e820. Ochsner, K.N., Silvers, J.A., Buhle, J.T., 2012. Functional imaging studies of emotion regulation: a synthetic review and evolving model of the cognitive control of emotion. Annals of the New York Academy of Sciences 1251, E1eE24. Ouyang, J., Carcea, I., Schiavo, J.K., Jones, K.T., Rabinowitsch, A., Kolaric, R., et al., 2017. Food restriction induces synaptic incorporation of calcium-permeable AMPA receptors in nucleus accumbens. European Journal of Neuroscience 45, 826e836. Palanza, P., Rodgers, R.J., Ferrari, P.F., Parmigiani, S., 1996. Effects of chlordiazepoxide on maternal aggression in mice depend on experience of resident and sex of intruder. Pharmacology Biochemistry and Behavior 54, 175e182. Parker, K.J., Buckmaster, C.L., Schatzberg, A.F., Lyons, D.M., 2004. Prospective investigation of stress inoculation in young monkeys. Archives of General Psychiatry 61, 933e941. Parker, K.J., Buckmaster, C.L., Sundlass, K., Schatzberg, A.F., Lyons, D.M., 2006. Maternal mediation, stress inoculation, and the development of neuroendocrine stress resistance in primates. Proceedings of the National Academy of Sciences of the United States of America 103, 3000e3005. Parker, K.J., Rainwater, K.L., Buckmaster, C.L., Schatzberg, A.F., Lindley, S.E., Lyons, D.M., 2007. Early life stress and novelty seeking behavior in adolescent monkeys. Psychoneuroendocrinology 32, 785e792. Poulton, R., Milne, B.J., Craske, M.G., Menzies, R.G., 2001. A longitudinal study of the etiology of separation anxiety. Behaviour Research and Therapy 39, 1395e1410. Rauhut, A.S., Thomas, B.L., Ayres, J.J., 2001. Treatments that weaken pavlovian conditioned fear and thwart its renewal in rats: implications for treating human phobias. Journal of Experimental Psychology: Animal Behavior Processes 27, 99e114. Rumpel, S., LeDoux, J., Zador, A., Malinow, R., 2005. Postsynaptic receptor trafficking underlying a form of associative learning. Science 308, 83e88. Russo, S.J., Murrough, J.W., Han, M.H., Charney, D.S., Nestler, E.J., 2012. Neurobiology of resilience. Nature Neuroscience 15, 1475e1484. Rutter, M., 2013. Annual research review: resilience–clinical implications. Journal of Child Psychology and Psychiatry 54, 474e487.

44

3. Learning to cope builds resilience

Sapolsky, R.M., 2015. Stress and the brain: individual variability and the inverted-U. Nature Neuroscience 18, 1344e1346. Saunders, T., Driskell, J.E., Johnston, J.H., Salas, E., 1996. The effect of stress inoculation training on anxiety and performance. Journal of Occupational Health Psychology 1, 170e186. Schmidt, M.V., Trumbach, D., Weber, P., Wagner, K., Scharf, S.H., Liebl, C., Datson, N., Namendorf, C., Gerlach, T., Kuhne, C., Uhr, M., Deussing, J.M., Wurst, W., Binder, E.B., Holsboer, F., Muller, M.B., 2010. Individual stress vulnerability is predicted by short-term memory and AMPA receptor subunit ratio in the hippocampus. Journal of Neuroscience 30, 16949e16958. Seery, M.D., Holman, E.A., Silver, R.C., 2010. Whatever does not kill us: cumulative lifetime adversity, vulnerability, and resilience. Journal of Personality and Social Psychology 99, 1025e1041. Serino, S., Triberti, S., Villani, D., Cipresso, P., Gaggioli, A., Riva, G., 2014. Toward a validation of cyber-interventions for stress disorders based on stress inoculation training: a systematic review. Virtual Reality 18, 73e87. Sotgiu, I., Mormont, C., 2008. Similarities and differences between traumatic and emotional memories: review and directions for future research. Journal of Psychology 142, 449e469. Staddon, J.E.R., 2016. Adaptive Behavior and Learning, second ed. Cambridge University Press, United Kingdom. Stetz, M.C., Thomas, M.L., Russo, M.B., Stetz, T.A., Wildzunas, R.M., McDonald, J.J., Wiederhold, B.K., Romano Jr., J.A., 2007. Stress, mental health, and cognition: a brief review of relationships and countermeasures. Aviation Space and Environmental Medicine 78 (5 Suppl. l), B252eB260. Storsve, A.B., McNally, G.P., Richardson, R., 2010. US habituation, like CS extinction, produces a decrement in conditioned fear responding that is NMDA dependent and subject to renewal and reinstatement. Neurobiology of Learning and Memory 93, 463e471. Tomita, S., Shenoy, A., Fukata, Y., Nicoll, R.A., Bredt, D.S., 2007. Stargazin interacts functionally with the AMPA receptor glutamate-binding module. Neuropharmacology 52, 87e91. Tritsch, N.X., Sabatini, B.L., 2012. Dopaminergic modulation of synaptic transmission in cortex and striatum. Neuron 76, 33e50. Ungar, M., 2015. Practitioner review: diagnosing childhood resilienceea systemic approach to the diagnosis of adaptation in adverse social and physical ecologies. Journal of Child Psychology and Psychiatry 56, 4e17. Vandenberghe, W., Nicoll, R.A., Bredt, D.S., 2005. Stargazin is an AMPA receptor auxiliary subunit. Proceedings of the National Academy of Sciences of the United States of America 102, 485e490. Wang, S.H., Tse, D., Morris, R.G., 2012. Anterior cingulate cortex in schema assimilation and expression. Learning and Memory 19, 315e318. Waugh, C.E., Koster, E.H., 2015. A resilience framework for promoting stable remission from depression. Clinical Psychology Review 41, 49e60. Weible, A.P., 2013. Remembering to attend: the anterior cingulate cortex and remote memory. Behavioural Brain Research 245, 63e75. Wolf, M.E., 2010. Regulation of AMPA receptor trafficking in the nucleus accumbens by dopamine and cocaine. Neurotoxicity Research 18, 393e409.

C H A P T E R

4

The brain mineralocorticoid receptor: a resilience factor for psychopathology? R. Angela Sarabdjitsingh1, E. Ron de Kloet2, Marian Joëls1, 3, Christiaan H. Vinkers4 1

Department of Translational Neuroscience, UMC Utrecht Brain Center, University Medical Center Utrecht, University of Utrecht, Utrecht, The Netherlands; 2Department of Endocrinology, Leiden University Medical Center, Leiden, The Netherlands; 3University of Groningen/University Medical Center Groningen, Groningen, The Netherlands; 4VU University Medical Center, Amsterdam, The Netherlands

The brain mineralocorticoid receptor Individuals need to adapt to a continuously changing internal and external environment to survive. Threats subjectively experienced in a changing environment, that is, stress, give rise to a response that is highly conserved among mammals. The stress response involves, among other things, a rapid activation of the locus coeruleus noradrenergic network and the its peripheral sympathetic adrenomedullar system resulting in increased circulating levels of adrenaline, and a slightly later activation of the hypothalamus-pituitary-adrenal (HPA) axis, which causes secretion of cortisol and/or corticosterone from the adrenal cortex, on top of the natural ultradian corticosteroid secretion pattern (Lightman and Conway-Campbell, 2010). These stress-induced waves of (nor)adrenaline and corticosteroids reach many organs including the brain, though with different kinetic properties: at least in rodents, corticosteroid hormones reach brain cells with a delay of 20 min compared with noradrenaline (Reul and De Kloet, 1985; Reul et al., 2015). The extent to which brain cells respond to these waves of hormones depends on many factors; for instance, corticosteroids’ access to the brain and neurons (and hence their response) depends on the expression of p-glycoproteins on epithelial or neural cells (Karssen et al., 2001; Pariante, 2008).

Stress Resilience https://doi.org/10.1016/B978-0-12-813983-7.00004-5

45

Copyright © 2020 Elsevier Inc. All rights reserved.

46

4. The brain mineralocorticoid receptor: a resilience factor for psychopathology?

A major factor determining exactly how neurons respond to waves of hormones is the availability of receptors, e.g., for (nor)adrenaline and corticosteroids. Multiple adrenoceptor subtypes are involved in the stress response, but a particularly prominent role is played by the b-adrenoceptor (De Quervain et al., 2017). Corticosterone and cortisol bind to two receptor types in the brain (de Kloet et al., 2005), that is, the high-affinity mineralocorticoid receptor (MR), in a limited number of limbic areas and motor nuclei; and the lower-affinity, more ubiquitously expressed glucocorticoid receptor (GR). Importantly, most MRs in brain differ from MRs in epithelial cells in kidney tubules, the bladder, intestines, salivary glands, and vascular endothelial cells, which all are aldosterone selective. Aldosterone selectivity in these cells primarily results from the activity of 11b-hydroxysteroid dehydrogenase type 2 (HSD-2), which converts the bioactive glucocorticoids to their 11-keto inactive congeners, thus allowing access of the less prevalent hormone aldosterone. In the brain, aldosterone-selective MRs do occur; they are localized in the nucleus tractus solitarii (NTS), some discrete cell groups in the medial amygdala and hypothalamus, and in the circumventricular organs (Geerling and Loewy, 2009; de Kloet and Joëls, 2017). The aldosterone-selective MR is involved in salt appetite, autonomous outflow, and volume regulation. However, most MR-expressing cells in the forebrain express 11b-HSD1 rather than 11b-HSD2, which actually regenerates bioactive glucocorticoids (Chapman et al., 2013) and therefore primarily bind the more abundant hormones corticosterone/cortisol rather than aldosterone.

Mineralocorticoid receptor activation and neuronal activity At the single cell level, the response to a stress-induced surge of noradrenaline and corticosteroids very much depends on the cellular expression pattern of receptors. With regard to corticosteroids, cells abundantly expressing both receptor types, such as hippocampal CA1 cells, show MR mediated effects that are generally found to be opposite from those mediated by GR (Joels et al., 2012). Activation of the high-affinity MR (already achieved with low doses of corticosteroids) is gradually counteracted by activation of the loweraffinity GR. In the majority of brain cells, however, GR is expressed at a substantially higher level than MR, which most likely results in a linear dose dependency (Joëls, 2006). Dentate granule cells (and possibly CA3 hippocampal cells) are an exception to these response patterns, because in these cells MR-mediated actions are efficiently induced, whereas GR-mediated actions are less effective, which causes a sigmoid dose-dependency curve. The brain’s response after stress is thus a composite of regional effects, partially dependent on the MR relative to GR expression. In addition to these regional differences, temporal aspects also play a role in the response to stress. Both MR and GR are located intracellularly and upon activation translocate to the nucleus, where they bind to response elements in 1%e2% of all genes (Datson et al., 2008; Gray et al., 2017). They act as slow transcriptional regulators, an effect that is fine-tuned by locally expressed cofactors (Zalachoras et al., 2013). However, over the past years, it has become increasingly evident that MR and GR can also mediate rapid nongenomic actions (Joels et al., 2012; Jiang et al., 2014). Most likely, this involves the same pool of receptors of which a small part moves to the vicinity of the plasma membrane and there mediates rapid effects (Karst et al., 2005), although convincing proof lacks to date.

Mineralocorticoid receptors and cognitive function in rodents

47

In the hippocampus and amygdala, areas that play an important role in the etiology of depression, MR induces rapid effects on glutamate transmission with an apparent lower affinity than the gene-mediated actions, rendering the rapid effects relevance during the stress response (Karst et al., 2005, 2010). Rapid MR effects occur in a time window where b-adrenoceptors are also active. Synergistic effects between the two hormones have been described, for example, in the dentate gyrus (Joels et al., 2012). A more complex interaction might take place in the basolateral amygdala (BLA). Here, rapid corticosteroid actions strongly depend on the recent stress history of the organism; that is, recent exposure of BLA neurons to either noradrenaline or corticosterone causes opposite effects to those seen in “naïve” BLA cells, a phenomenon called metaplasticity (Karst et al., 2010). Waves of the b-adrenoceptor agonist isoproterenol and corticosterone revealed BLA responses ranging from a transient excitation followed by inhibition (with low hormone concentrations, as seen during very mild stress) to prolonged excitatory responsesdinduced by very high doses of the hormones, as may occur during severe stress (see Fig. 4.1; Karst and Joëls, 2016). All in all, the stress response in the brain forms a composite of regionally and temporally diverse responses, depending on the local concentration of noradrenaline, corticosterone and their receptors.

Mineralocorticoid receptors and cognitive function in rodents MR and GR are both widely expressed in brain areas that are important for learning and memory processes and promote behavioral adaptation in a distinct yet highly coordinated manner (Joëls and Baram, 2009; Harris et al., 2013). Over the past decades, accumulating evidence has especially implicated MR in emotional and cognitive control by appraisal of novel situations, learning strategies, and response selection (Vogel et al., 2016). For example, early studies using icv administration of a selective MR antagonist, albeit on a background of adrenalectomized animals, pointed to involvement of MR in search-escape strategies and behavioral reactivity to spatial novelty in rats (Oitzl and de Kloet, 1992; Oitzl et al., 1994). MR also plays a role in contextual and tone-cue formation and appraisal of situations, as this is negatively affected when mice are treated with spironolactone, an MR antagonist, prior to training (Zhou et al., 2011). Another line of studies showed that corticosteroid hormones, through MR, promote stressinduced switching in spatial strategy formation (Schwabe et al., 2010; ter Horst et al., 2013b). Application of spironolactone was reported to block the switch from hippocampal to the cognitive less demanding striatal-based habit learning, resulting in poor performance in mice (Schwabe et al., 2010, 2013a,b; ter Horst et al., 2014; Arp et al., 2014). In addition to these pharmacological studies, genetically modified animal models have also proven to be very useful in delineating the function of MR. Several rodent models have been described using either a genetically modified or viral modification strategy to specifically target MR in the brain. For instance, MR-deficient mice showed impaired performance in learning and memory tasks, behavioral flexibility, and switching between behavioral strategies in the radial and water maze and circular hole board (Berger et al., 2006; ter Horst et al., 2013a; Schwabe et al., 2010; Brinks et al., 2009). Conversely, in rats with HSV viral vectoremediated overexpression of MR or in transgenic MR overexpression micee enhanced memory performance, faster behavioral adaptation and reduced anxiety was found

48

4. The brain mineralocorticoid receptor: a resilience factor for psychopathology?

Frequency (Hz)

Frequency (Hz)

10 9 8 7 6 5 4 3 2 1 0

“very mild stress”

10 9 8 7 6 5 4 3 2 1 0

“moderate stress”

Frequency (Hz)

10 8 6 4 2

“severe stress”

0

FIGURE 4.1 Cellular responses of basolateral amygdala neurons to waves of stress hormones. Basolateral amygdala cells in vitro were exposed to waves of first isoproterenol (green) and next corticosterone (orange), mimicking the natural variations measured with microdialysis. The top panel shows a brief wave of 0.3 mM isoproterenol (mimicking very mild stress), the middle panel waves of 1 mM isoproterenol followed by 30 nM corticosterone (mimicking moderate stress), and the lower panel the application of 3 mM isoproterenol followed by 100 nM corticosterone (severe stress). The graphs show the averaged (þSEM) frequency of miniature excitatory postsynaptic currents in time. The intensity of the bar’s color corresponds to the significance of the effect; red bars indicate excitatory responses and blue bars indicate inhibitory responses. The difference between very mild and moderate stress is characterized by the appearance of a brief excitatory response, whereas the shift from moderate to severe stress is associated with the appearance of a delayed excitatory effect. Based on Karst, H. Joëls, M., 2016. Severe stress hormone conditions cause an extended window of excitability in the mouse basolateral amygdala. Neuropharmacology 110, 175e180. https://doi.org/10.1016/j.neuropharm.2016.07.027.

Pharmacology, genetic variation, and vulnerability to psychopathology in humans

49

(Lai et al., 2007; Rozeboom et al., 2007; Ferguson and Sapolsky, 2008; Mitra et al., 2009). When a mouse line with overexpression of MR was combined with decreased GR, a similar behavioral phenotype was found including improved spatial memory and perseverance of a learned behavioral response (Harris et al., 2013). All in all, the MR appears involved in increased attention and vigilance in anticipation of upcoming events, appraisal of novel information, and retrieval of previously acquired behavioral response patterns, to appropriately deal with the stressor. Moreover, activation of the MR facilitates encoding of the experience to facilitate learning processes. These initial physiological and behavioral reactions to novelty are crucial for the onset of the stress reaction. The impact of MR (and GR) on cognitive performance depends on not only the genetic background but also its interaction with environmental influences, especially influences taking place early in life. Alterations in maternal care are thought to partly mediate the effects of early-life adversity on diminished functioning of MRs and GRs (Liu et al., 1995; Weaver et al., 2004; Champagne et al., 2008). Several studies suggest that high expression of MR may be beneficial in promoting resilience after chronic or early-life stress (ELS). For instance, ELS or chronic unpredictable stress in adulthood reduces hippocampaldependent contextual learning while enhancing fear learning (Kanatsou et al., 2015, 2017). Forebrain-specific overexpression of MR in mice (partially) prevented this effect on contextual memory formation, most likely by impacting on neurogenesis and synaptic transmission in dentate granule cells in the hippocampus. Pharmacological studies have the advantage that the window of application of (ant)agonists can be precisely timed. Treatment with the GR antagonist mifepristone (RU38486) is a relatively simple strategy, as it may reset or shift the MR:GR balance by (temporarily) inactivating GR while enhancing MR function. For instance, in mice, ELS enhances freezing behavior in-between conditioned cue exposure in fear learning, which is ameliorated by brief blockade of GR during the critical developmental window of adolescence (Arp et al., 2016). Similarly in rats, mifepristone (administered during early adolescence) was reported to normalize hippocampus striataledependent contextual memory and spatial learning deficits after maternal deprivation, possibly by impacting on the glutamatergic neurotransmission system (Loi et al., 2017b) (see Fig. 4.2). This effect was specifically found in males (Loi et al., 2017a). Yet, not all behavioral domains may be sensitive to mifepristone treatment during early adolescence, because ELS-induced deficits in behavioral inhibition and attention as measured by the five-choice serial reaction time task were not normalized (Kentrop et al., 2016). Altogether, preclinical research has provided valuable knowledge on the in vivo functions of MR in the brain and generally suggests that this receptor may be protective for stress effects on brain function, especially prolonged or severe stress early in life.

Pharmacology, genetic variation, and vulnerability to psychopathology in humans The mineralocorticoid and hypothalamus-pituitary-adrenal axis activity As previously demonstrated in rodents by pharmacological and genetic approaches (Ratka et al., 1989; Harris et al., 2013), there is also evidence that MR affects basal and stress-induced

50

4. The brain mineralocorticoid receptor: a resilience factor for psychopathology?

(A) PND 3

PND 26-28

24h maternal deprivaon

GR antagonist (mifepristone)

(B)

adult behavioral tesng

(C)

habituaon (d1)

learning trial I (d2)

learning trial II (d2)

test trial (d3)

FIGURE 4.2 Effect of early-life stress on contextual memory and the possibility to intervene with mifepristone treatment during early puberty. (A) Timeline of the experiment. (B) Setup of the object-in-context experiment. Male rats were initially habituated in a context that had no object. Next, during training, rats were placed in the same context but with two identical objects (learning trial I) and then placed in a novel context with two identical novel objects (learning trial II). Finally the rats were placed in the latter context but with one object being replaced by an object from the first context (test trial). (C) The discrimination index as observed from the object in context experiment. Rats that earlier had been exposed to maternal deprivation showed impaired discrimination between the objects. This was fully restored in animals that had been treated with mifepristone between days 26 and 28. Data expressed as mean  SEM. Posthoc testing: *P < .05; **P < .01. Based on Loi, M., Sarabdjitsingh, R.A., Tsouli, A., Trinh, S., Arp, M., Krugers, H.J., Karst, H., van den Bos, R., Joëls, M., November 2, 2017a. Transient prepubertal mifepristone treatment normalizes deficits in contextual memory and neuronal activity of adult male rats exposed to maternal deprivation. eNeuro 4 (5). pii: ENEURO.0253-17.2017. https://doi.org/10.1523/ENEURO.0253-17.2017. eCollection 2017 Sep-Oct; Loi, M., Mossink, J.C.L., Meerhoff, G.F., Den Blaauwen, J.L., Lucassen, P.J., Joëls, M., 2017b. Effects of early-life stress on cognitive function and hippocampal structure in female rodents. Neuroscience 342, 101e119. https://doi.org/10.1016/j.neuroscience.2015. 08.024.

HPA axis activity in humans. First, MR antagonists such as canrenoate (Arvat et al., 2001; Wellhoener et al., 2004) and spironolactone (Cornelisse et al., 2011; Deuschle et al., 1998; Otte et al., 2007; Young et al., 1998) increase basal cortisol levels. Vice versa, fludrocortisone, a potent MR agonist, inhibits the HPA axis and subsequently decreases basal cortisol levels (Otte et al., 2003). However, because fludrocortisone is also a quite potent GR agonist, these data are somewhat difficult to interpret. These insights obtained with pharmacological approaches are supported when studying the effect of genetic variation in MR (de Kloet et al., 2016) (see Fig. 4.3). The MR SNP rs2070951 influences basal cortisol levels and the cortisol awakening response with higher basal cortisol levels in G-carriers but lower levels in C-carriers (van Leeuwen et al., 2010a; Muhtz et al., 2011; Klok et al., 2011a,b,c). In male twins, Val-carriers of I180V consistently displayed an increased cortisol stress response after repeated exposure to the Trier Social Stress Test (DeRijk et al., 2006), but not all studies have found significant stress effects for these individual MR SNPs (Bouma et al., 2011; Ising et al., 2008).

Pharmacology, genetic variation, and vulnerability to psychopathology in humans

51

FIGURE 4.3 The human MR gene and two major haplotype blocks. Introns are indicated in gray and the exons (1b, 1a, and 2e9) in color. P2 and P1 are promoter regions. UTR ¼ untranslated region. In the 50 region, three common functional haplotypes have been described, based on two SNP, MR-2C/G (rs2070951) and MRI180V (rs5522). In the 30 region (exon 9), a second haplotype block is present based on rs5534 and rs2871 from which three (other) common haplotypes can be constructed. Frequencies of the haplotypes are indicated. Based on ter Heegde, F., De Rijk, R.H., Vinkers, C.H., February 2015. The brain mineralocorticoid receptor and stress resilience. Psychoneuroendocrinology 52, 92e110. https://doi.org/10.1016/j.psyneuen.2014.10.022. Epub 2014 Nov 7.

The MR variants rs5522 and rs2070951 are in low linkage disequilibrium and inherited as haplotypes. The CA combination (haplotype 2) resulted in vitro in much higher MR expression and transactivation than GA (haplotype 1). Interestingly, in a group of school teachers, the CA MR haplotype 2 was associated with a higher cortisol, ACTH, and heart rate response following stress (van Leeuwen et al., 2011). The GA MR haplotype carriers showed the most pronounced sex difference in suppression of the CAR. Moreover, the male GA carriers with the highest CAR also showed the strongest resistance to dexamethasone suppression (van Leeuwen et al., 2010a). Altogether, both pharmacological and genetic evidence support a role for MR in the regulation of the HPA axis, the exact nature of which may be related to (1) differences in sex, age, and contraceptive use; (2) unaccounted variation by trauma exposure and a history of psychiatric disorders; and (3) methodological differences (e.g., the use of single SNPs vs. haplotypes).

The mineralocorticoid, learning, and stress appraisal in humans The MR is also important for stress-related memory processes in humans, because the MR antagonist spironolactone affects different aspects of stress-related memory formation. In healthy men, administration of spironolactone prior to stress exposure (but not prior to the control condition) resulted in short-term working memory impairments yet enhanced longterm memory (Cornelisse et al., 2011). Whether these effects result from MR blockade per se or a relative increase in GR activation cannot be delineated from these pharmacological studies. Spironolactone also prevented stress-induced increases in response inhibition (Schwabe et al., 2013a) as well as a switch from hippocampus-based learning to striatumor amygdala-based learning approach (Schwabe et al., 2013b; Vogel et al., 2015, 2017). In support, Val-allele carriers of rs5522 showed an impaired stress-induced reward learning (Bogdan et al., 2010), suggesting that lower MR functionality may be detrimental for stress-related learning. Moreover, carriers of the MR haplotype 2 display an improved shift from cognitive to striatal habit learning (Wirz et al., 2017).

52

4. The brain mineralocorticoid receptor: a resilience factor for psychopathology?

The MR also impacts on learning strategies under nonstressful circumstances: the MR antagonist spironolactone impaired selective attention under nonstressful conditions (Otte et al., 2007), whereas the MR agonist fludrocortisone improved verbal memory in both healthy controls and depressed patients (Otte et al., 2015). The MR also facilitates stress appraisal by an appropriate evaluation of the context, with genetic variation in the MR exon SNPs rs5534/rs2871 (loss-of-function) being associated with negative memory bias, particularly after exposure to early-life adversity (Vogel et al., 2014). This was interpreted as evidence that lower MR activity may enhance memory formation for sad and pessimistic stimuli, emphasizing that MR functionality is important for contextual recognition and subsequent appraisal of stress. Indeed, Val-carriers of rs5522 showed a greater threatrelated amygdala reactivity (Bogdan et al., 2012; Kuningas et al., 2007).

The mineralocorticoid receptor and resilience and vulnerability for psychiatric disorders With a role of the MR in HPA axis activity, memory, and appraisal, under both basal and stressful circumstances, it may be hypothesized that the MR is also involved in the onset and course of stress-related psychiatric disorders. Indeed, blunted or excessive HPA axis activity has been repeatedly linked to anxiety and mood disorders (Spijker and Van Rossum, 2012). Postmortem studies investigating MR expression in the human brain have shown remarkably consistent results, with major depressive disorder (MDD) brains showing lower MR mRNA expression in the hippocampus (Klok et al., 2011a,b,c; Medina et al., 2013), inferior frontal gyrus, and cingulate gyrus (Klok et al., 2011a,b,c) and lower hippocampal MR expression in suicide victims (Young et al., 1998). Similarly, schizophrenia and bipolar disorder patients displayed lower MR mRNA levels in the dorsolateral prefrontal cortex (Qi et al., 2013; Xing et al., 2004). One study found higher MR expression in the hypothalamic paraventricular nucleus in MDD, which may be compensatory to reduced hippocampal and cortical MR expression (Wang et al., 2008). Lower MR expression in limbic areas would reduce the tonic inhibition of the HPA axis and subsequently increase the risk for mood disorders. In support, data from preclinical studies show that various classes of antidepressants (among which MAO inhibitors, selective serotonin reuptake inhibitors (SSRI), and tricyclic antidepressants [TCA]) consistently increase hippocampal MR expression in rodents (Bjartmar et al., 2000; Lopez et al., 1998; Reul et al., 1994; Seckl and Fink, 1992; Yau et al, 1995). This effect depends, in part, on the duration of antidepressant treatment (Yau et al., 2001). There are several studies linking genetic MR variation to susceptibility for psychiatric disorders. MR haplotype 2 is associated with enhanced resilience to depression in females (Klok et al., 2011a,b,c) and a higher dispositional optimism with fewer thoughts of hopelessness (Klok et al., 2011a,b,c). In agreement, Val-allele carriers of rs5522 (with decreased MR activity) displayed increased vulnerability for depressive symptoms in elderly individuals (Kuningas et al., 2007). Moreover, MR haplotype 2 sex-dependently moderated the relation between childhood maltreatment and depressive symptoms both in a population-based sample and in a clinical sample (Vinkers et al., 2015). This is supported by a recent study

Concluding remarks

53

showing a pleiotropic interaction between childhood trauma and the MR on cortisol levels and stress-related phenotypes (Gerritsen et al., 2017). These data suggest that increased MR activity could constitute a possible treatment for MDD. Indeed, the MR agonist fludrocortisone, as add-on treatment to the SSRI escitalopram in a double-blind placebo-controlled randomized clinical trial with 64 MDD patients, resulted in an accelerated escitalopram response in the responder group (Otte et al., 2010). This corresponds with earlier findings that the MR antagonist spironolactone decreased the antidepressant effects of the TCA amitriptyline (Holsboer, 1999). Furthermore, add-on treatment with metyrapone, a cortisol synthesis inhibitor, enhanced effectiveness of antidepressants, which could be partially due to metyrapone-induced MR upregulation (Jahn et al., 2004). Interestingly, impaired MR function may predict nonresponse to antidepressants, with SSRI responders showing an intact MR functionality following the prednisolone suppression test (Juruena et al., 2006, 2009). Preclinical studies, however, argued against beneficial effects of mineralocorticoid activation if aldosterone-selective MRs are excessively stimulated, because chronic aldosterone treatment in rats induced anxiety and depressivelike phenotypes (Hlavacova and Jezova, 2008; Hlavacova et al., 2012). This suggests that excessive aldosterone-selective MR stimulation leads to an increased vulnerability for anxiety and depression (Murck et al., 2012). Moreover, a complicating factor in using the MR as a drug target for depression is its potential cardiac proinflammatory effect (Rafatian et al., 2014) (see Concluding remarks section). In summary, evidence for a role of the MR in psychiatric disorders stems from postmortem studies, pharmacological manipulations, genetic studies, and treatment studies. These findings suggest that in healthy individuals, high (but not supraphysiologically high) compared with low MR functionality may be related to resilience to MDD. Yet, large and methodologically sound studies investigating MR-related compounds are still lacking.

Concluding remarks Brain mineralocorticoids important for resilience? As argued, MRs expressed in the limbic brain play a crucial role in mediating the action of naturally occurring glucocorticoids in coping with stress. This action exerted by corticosterone and cortisol involves the role of the receptor in the HPA axis itself: MR is important for the tone and threshold of the HPA axis responsiveness, which is revealed upon systemic, intracerebroventricular, or hippocampal application of MR antagonists (De Kloet et al., 1998; Joëls and de Kloet, 2017). MR also plays a role in several cognitive domains and is accompanied by emotional expressions of, for example, fear and aggression. Thus, MR activation results in increased attention and vigilance in anticipation of upcoming events, appraisal of novel information, and retrieval of previously acquired behavioral response patterns to deal appropriately with the stressor. Activation of the MR also promotes encoding of the experience to facilitate learning processes. Collectively, these initial physiological and behavioral reactions to novelty are important for the onset of the stress reaction (De Kloet et al., 2005; Joëls and de Kloet, 2017).

54

4. The brain mineralocorticoid receptor: a resilience factor for psychopathology?

The rise in cortisol and corticosterone that marks the onset of the stress reaction then proceeds to additionally occupy the lower-affinity GR, which in many limbic cells is colocalized with the MR. The GR mediates the action of corticosterone and cortisol, in a complementary manner to MR. Thus, MR activation initiates primary defense reactions, which are subsequently suppressed via GR with the goal to prevent these initial responses from overshooting and to become damaging themselves (Sapolsky et al., 2000; De Kloet et al., 2005). Increased excitability is suppressed by GR activation, and behavioral adaptation and fading of the concomitant stress-induced HPA axis activity are facilitated. Meanwhile, GR activation primes brain circuits to allow the individual to cope with similar challenges in future: this implies contextualization and memory consolidation of the selected coping strategy and its concordant emotional expressions. To what extent can activation of MRs contribute to resilience? Resilience has been defined by the American Psychological Association as “the process of adapting well in the face of adversity, trauma, tragedy, threats or even significant sources of stress.” Resilience is the ability to learn from a stressful challenge and to grow in the face of adversity, a process that metaphorically has been labeled “to bounce back.” As pointed out by Southwick et al. (2014), resilience is not absent or present but represents a continuum, a complex construct that is variable in different contexts, shows a large individual variation, and can change over lifetime. Self-esteem, social support, and the socioeconomic status are some of the important determinants of resilience in coping with stress. When exposed to a stressor, a resilient individual is able to rapidly switch on its HPA axis and cortisol secretion, as long as it also efficiently terminates stress hormone release. This is precisely what does not happen during lack of control and uncertainty: rhythmicity is flattened and the neuroendocrine stress response develops slowly and persists (McEwen et al., 2015). Stress mediators, such as noradrenaline or corticosterone/cortisol, are of obvious significance for understanding the mechanism of resilience. We propose that a proper balance in signaling cascades that regulate physiological responses and behavioral adaptation to a stressor is actually the key to resilience. Thus, for optimal resilience, the sympathetic and parasympathetic nervous system, the pro- and antiinflammatory cytokines, and the activating and inhibiting arms of the HPA axis need to be in balance. In the HPA axis, this balance is represented by the CRF/vasopressin/MR drive, which is balanced in time by a GR-mediated feedback. Supraphysiological activation of MR (relative to GR) is therefore expected to be as disadvantageous as conditions that lead to a strong reduction in MR function (De Kloet, 2014). Interestingly, Selye (1950) distinguished glucocorticoids and mineralocorticoids as opposing regulators in his pendulum hypothesis: the prophlogistic mineralocorticoids increase the risk for inflammation and the anti-phlogistic glucocorticoids cause risk for infection. As pointed out in The brain mineralocorticoid receptor section, in the brain the complementary MR- and GR-mediated actions are actually embodied by one single class of hormones: the naturally occurring glucocorticoids cortisol and corticosterone. Current evidence suggests that low MR functionality may be a predisposing factor to develop psychopathology, especially in women. The sex dependency may be related to the promiscuity of the MR, as the receptor also responds to progesterone, acting as an antagonist. Hence, the rising progesterone levels during the cycle or during contraceptive use inhibit MR function and cause activation of the HPA axis as well as change the threshold for emotional expressions (Carey et al., 1995). Remarkably, rodent studies show stronger phenotypes in

Concluding remarks

55

male subjects, both of early-life adversity and interventions targeting the MR. It should be noted, however, that most studies in rodents are carried out in male subjects to start with, so that the amount of information on females is more restricted. The role of brain MRs through modulation of cognitive processing cannot be regarded as independent from other physiological processes in which the receptor is involved. For instance, the aldosterone-selective MR neurons of the NTS innervate forebrain regions that express the nonselective MR, that is, the receptor that responds to glucocorticoids to regulate cognitive and emotional aspects and also the mesolimbic cortical dopaminergic pathways arising from the ventral tegmental A10 cell group (Geerling et al., 2006; de Kloet and Joëls, 2017). It is thought that this circuit mediates the high motivation of salt-depleted animals to search for salt and, in case of excess, rapidly can switch the taste of salt to disgust. In recent experiments using the spontaneous hypertensive rats (SHRs), de Nicola’s group in Buenos Aires demonstrated a vicious chain of events that was initiated by the global overexpression of MR in this species in much the same way as shown by the classical DOCA-salt model. Hypertension causes vasculopathy, which leads to neuronal damage through hypoxia. At the same time, microglia proliferate, leading to the production of proinflammatory cytokines, which further aggravate a feedforward cascade of neurodegeneration (Brocca et al., 2017). Virtually every step is promoted by overactivity of the MR, which has been demonstrated with the proinflammatory cytokines (Schöbitz et al., 1994). A similar chain of events is at the basis of congestive heart disease and obesity in adipose tissue, suggesting a common basis for the comorbidity of cardiovascular, metabolic, and brain disease.

Future directions Many questions regarding the role of MRs in the brain are still open. First, the molecular mechanism of action most likely will resolve the enormous diversity in function displayed by the promiscuous MR. In their genomic mode of action, this includes the role of heterodimerization of MR:GR, the function of the neuroD transcription factors to assure MR specificity and the context-dependent amplification in function via coregulators (Lachize et al., 2009; Van Weert et al., 2017). How these slow genomic actions are integrated with the rapid membrane-mediated MR actions is another mystery awaiting resolution. Second, the gain in function MR haplotype 2 invariably predicts mental health in females (Klok et al., 2011a,b,c; Vinkers et al., 2015; Hamstra et al., 2017). Yet, genetic MR variants have been intensely studied so far only by a limited number of investigators, which calls for replication. How early-life experience modifies MR function by epigenetic pathways is another question that awaits an answer. Understanding when, where, and how such epigenetic programming occurs might help to predict a mechanism of resilience later in life; in other words, to better understand the mechanistic underpinning of the adaptive capacity by asking how early-life experience organizes the brain and body function for life to come and what role the MR:GR balance plays in programming (Daskalakis et al., 2013). And finally: What is the mechanism causing the switch from a life-sustaining MR to a “proverbial” death receptor? (Brocca et al., 2017). It seems that boosting MR function under healthy conditions preserves health, but during persistent adversitydas demonstrated with

56

4. The brain mineralocorticoid receptor: a resilience factor for psychopathology?

proinflammatory feedforward cascade in the brain of hypertensivesdinappropriately high MR would demand treatment with MR antagonists as a lifesaver. These actions mediated by MR cannot be seen in isolation from its GR companion, because the functioning of both receptor types needs to be in balance to preserve homeostasis, resilience, and health.

References Arp, J.M., Ter Horst, J.P., Kanatsou, S., Fernández, G., Joëls, M., Krugers, H.J., Oitzl, M.S., 2014. Mineralocorticoid receptors guide spatial and stimulus-response learning in mice. PLoS One 9 (1). https://doi.org/10.1371/ journal.pone.0086236. Arp, J.M., ter Horst, J.P., Loi, M., den Blaauwen, J., Bangert, E., Fernández, G., et al., 2016. Blocking glucocorticoid receptors at adolescent age prevents enhanced freezing between repeated cue-exposures after conditioned fear in adult mice raised under chronic early life stress. Neurobiology of Learning and Memory 133, 30e38. https://doi.org/10.1016/j.nlm.2016.05.009. Arvat, E., Maccagno, B., Giordano, R., Pellegrino, M., Broglio, F., Gianotti, L., et al., 2001. Mineralocorticoid receptor blockade by canrenoate increases both spontaneous and stimulated adrenal function in humans. Journal of Clinical Endocrinology and Metabolism 86. https://doi.org/10.1210/jcem.86.7.7663. Berger, S., Wolfer, D.P., Selbach, O., Alter, H., Erdmann, G., Reichardt, H.M., et al., 2006. Loss of the limbic mineralocorticoid receptor impairs behavioral plasticity. Proceedings of the National Academy of Sciences of the United States of America 103 (1), 195e200. https://doi.org/10.1073/pnas.0503878102. Bjartmar, L., Johansson, I.M., Marcusson, J., Ross, S.B., Seckl, J.R., Olsson, T., 2000. Selective effects on NGFI-A, MR, GR and NGFI-B hippocampal mRNA expression after chronic treatment with different subclasses of antidepressants in the rat. Psychopharmacology 151, 7e12. https://doi.org/10.1007/s002130000468. Bogdan, R., Perlis, R.H., Fagerness, J., Pizzagalli, D.A., 2010. The impact of mineralocorticoid receptor ISO/VAL genotype (rs5522) and stress on reward learning. Genes, Brain and Behavior 9 (6), 658e667. https://doi.org/ 10.1111/j.1601-183X.2010.00600.x. Bogdan, R., Williamson, D.E., Hariri, A.R., 2012. Mineralocorticoid receptor Iso/Val (rs5522) genotype moderates the association between previous childhood emotional neglect and amygdala reactivity. American Journal of Psychiatry 169 (5), 515e522. Bouma, E.M.C., Riese, H., Nolte, I.M., Oosterom, E., Verhulst, F.C., Ormel, J., Oldehinkel, A.J., 2011. No associations between single nucleotide polymorphisms in corticoid receptor genes and heart rate and cortisol responses to a standardized social stress test in adolescents: the TRAILS study. Behavior Genetics 41 (2), 253e261. https:// doi.org/10.1007/s10519-010-9385-6. Brinks, V., Berger, S., Gass, P., de Kloet, E.R., Oitzl, M.S., 2009. Mineralocorticoid receptors in control of emotional arousal and fear memory. Hormones and Behavior. https://doi.org/10.1016/j.yhbeh.2009.05.003. Brocca, M.E., Pietranera, L., Meyer, M., Lima, A., Roig, P., de Kloet, E.R., De Nicola, A.F., 2017. Mineralocorticoid receptor associates with pro-inflammatory bias in the hippocampus of spontaneously hypertensive rats. Journal of Neuroendocrinology. https://doi.org/10.1111/jne.12489. Carey, M.P., Deterd, C.H., De Koning, J., Helmerhorst, F., De Kloet, E.R., 1995. The influence of ovarian steroids on hypothalamic-pituitary-adrenal regulation in the female rat. Journal of Endocrinology. Retrieved from: http:// www.scopus.com/inward/record.url?eid¼2-s2.0-0028931878&partnerID¼MN8TOARS. Champagne, D.L., Bagot, R.C., van Hasselt, F., Ramakers, G., Meaney, M.J., de Kloet, E.R., Joëls, M., Krugers, H., 2008. Maternal care and hippocampal plasticity: evidence for experience-dependent structural plasticity, altered synaptic functioning, and differential responsiveness to glucocorticoids and stress. Journal of Neuroscience 28 (23), 6037e6045. Chapman, K., Holmes, M., Seckl, J., July 2013. 11b-hydroxysteroid dehydrogenases: intracellular gate-keepers of tissue glucocorticoid action. Physiological Reviews 93 (3), 1139e1206. https://doi.org/10.1152/physrev. 00020.2012. Cornelisse, S., Joëls, M., Smeets, T., 2011. A randomized trial on mineralocorticoid receptor blockade in men: effects on stress responses, selective attention, and memory. Neuropsychopharmacology 36 (13), 2720e2728. https://doi.org/10.1038/npp.2011.162.

References

57

Daskalakis, N.P., Bagot, R.C., Parker, K.J., Vinkers, C.H., de Kloet, E.R., 2013. The three-hit concept of vulnerability and resilience: toward understanding adaptation to early-life adversity outcome. Psychoneuroendocrinology. https://doi.org/10.1016/j.psyneuen.2013.06.008. Datson, N.A., Morsink, M.C., Meijer, O.C., de Kloet, E.R., 2008. Central corticosteroid actions: search for gene targets. European Journal of Pharmacology. https://doi.org/10.1016/j.ejphar.2007.11.070. De Kloet, E.R., 2014. From receptor balance to rational glucocorticoid therapy. Endocrinology. https://doi.org/10. 1210/en.2014-1048. de Kloet, E.R., Joëls, M., 2017. Brain mineralocorticoid receptor function in control of salt balance and stressadaptation. Physiology and Behavior. https://doi.org/10.1016/j.physbeh.2016.12.045. De Kloet, E.R., Joëls, M., Holsboer, F., June 2005. Stress and the brain: from adaptation to disease. Nature Reviews Neuroscience 6 (6), 463e475. de Kloet, E.R., Otte, C., Kumsta, R., Kok, L., Hillegers, M.H.J., Hasselmann, H., et al., 2016. Stress and depression: a crucial role of the mineralocorticoid receptor. Journal of Neuroendocrinology. https://doi.org/10.1111/jne.12379. De Kloet, E.R., Vreugdenhil, E., Oitzl, M.S., Joëls, M., 1998. Brain corticosteroid receptor balance in health and disease. Endocrine Reviews. Retrieved from: http://www.scopus.com/inward/record.url?eid¼2-s2.0-003 2467785&partnerID¼MN8TOARS. De Quervain, D., Schwabe, L., Roozendaal, B., January 2017. Stress, glucocorticoids and memory: implications for treating fear-related disorders. Nature Reviews Neuroscience 18 (1), 7e19. DeRijk, R.H., Wüst, S., Meijer, O.C., Zennaro, M.-C., Federenko, I.S., Hellhammer, D.H., et al., 2006. A common polymorphism in the mineralocorticoid receptor modulates stress responsiveness. Journal of Clinical Endocrinology and Metabolism 91, 5083e5089. https://doi.org/10.1210/jc.2006-0915. Deuschle, M., Weber, B., Colla, M., Müller, M., Kniest, A., Heuser, I., 1998. Mineralocorticoid receptor also modulates basal activity of hypothalamus-pituitary-adrenocortical system in humans. Neuroendocrinology 68, 355e360. https://doi.org/10.1159/000054384. Ferguson, D., Sapolsky, R., 2008. Overexpression of mineralocorticoid and transdominant glucocorticoid receptor blocks the impairing effects of glucocorticoids on memory. Hippocampus 18 (11), 1103e1111. https://doi.org/ 10.1002/hipo.20467. Geerling, J.C., Kawata, M., Loewy, A.D., 2006. Aldosterone-sensitive neurons in the rat central nervous system. The Journal of Comparative Neurology 494 (3), 515e527. https://doi.org/10.1002/cne.20808. Geerling, J.C., Loewy, A.D., 2009. Aldosterone in the brain. The Australian Journal of Pharmacy: Renal Physiology 297 (3), F559eF576. https://doi.org/10.1152/ajprenal.90399.2008. Gerritsen, L., Milaneschi, Y., Vinkers, C.H., van Hemert, A.M., van Velzen, L., Schmaal, L., Penninx, B.W., November 2017. HPA Axis genes, and their interaction with childhood maltreatment, are related to cortisol levels and stressrelated phenotypes. Neuropsychopharmacology 42 (12), 2446e2455. https://doi.org/10.1038/npp.2017.118. Gray, J.D., Kogan, J.F., Marrocco, J., McEwen, B.S., 2017. Genomic and epigenomic mechanisms of glucocorticoids in the brain. Nature Reviews Endocrinology. https://doi.org/10.1038/nrendo.2017.97. Hamstra, D.A., de Kloet, E.R., Quataert, I., Jansen, M., Van der Does, W., 2017. Mineralocorticoid receptor haplotype, estradiol, progesterone and emotional information processing. Psychoneuroendocrinology. https://doi.org/10. 1016/j.psyneuen.2016.11.037. Harris, A.P., Holmes, M.C., De Kloet, E.R., Chapman, K.E., Seckl, J.R., 2013. Mineralocorticoid and glucocorticoid receptor balance in control of HPA axis and behaviour. Psychoneuroendocrinology. https://doi.org/10.1016/j. psyneuen.2012.08.007. Hlavacova, N., Jezova, D., 2008. Chronic treatment with the mineralocorticoid hormone aldosterone results in increased anxiety-like behavior. Hormones and Behavior 54, 90e97. https://doi.org/10.1016/j.yhbeh.2008.02.004. Hlavacova, N., Wes, P.D., Ondrejcakova, M., Flynn, M.E., Poundstone, P.K., Babic, S., Murck, H., Jezova, D., 2012 Mar. Subchronic treatment with aldosterone induces depression-like behaviours and gene expression changes relevant to major depressive disorder. The International Journal of Neuropsychopharmacology 15 (2), 247e265. https://doi.org/10.1017/S1461145711000368. Epub 2011 Mar 4. Holsboer, F., 1999. The rationale for corticotropin-releasing hormone receptor (CRH-R) antagonists to treat depression and anxiety. Journal of Psychiatric Research. https://doi.org/10.1016/S0022-3956(98)90056-5. Ising, M., Depping, A.M., Siebertz, A., Lucae, S., Unschuld, P.G., Kloiber, S., et al., 2008. Polymorphisms in the FKBP5 gene region modulate recovery from psychosocial stress in healthy controls. European Journal of Neuroscience 28 (2), 389e398. EJN6332 [pii]. https://doi.org/10.1111/j.1460-9568.2008.06332.x.

58

4. The brain mineralocorticoid receptor: a resilience factor for psychopathology?

Jahn, H., Schick, M., Kiefer, F., Kellner, M., Yassouridis, A., Wiedemann, K., 2004. Metyrapone as additive treatment in major depression: a double-blind and placebo-controlled trial. Archives of General Psychiatry 61, 1235e1244, 61/12/1235 [pii]\n. https://doi.org/10.1001/archpsyc.61.12.1235. Jiang, C.L., Liu, L., Tasker, J.G., 2014. Why do we need nongenomic glucocorticoid mechanismsa. Frontiers in Neuroendocrinology. https://doi.org/10.1016/j.yfrne.2013.09.005. Joëls, M., 2006. Corticosteroid effects in the brain: U-shape it. Trends in Pharmacological Sciences 27 (5), 244e250. https://doi.org/10.1016/j.tips.2006.03.007. Joëls, M., Baram, T.Z., 2009. The neuro-symphony of stress. Nature Reviews Neuroscience. https://doi.org/10.1038/ nrn2632. Joëls, M., de Kloet, E.R., 2017. The brain mineralocorticoid receptor: a saga in three episodes. Journal of Endocrinology 234 (1), T49eT66. https://doi.org/10.1530/JOE-16-0660. Joels, M., Sarabdjitsingh, R.A., Karst, H., 2012. Unraveling the time domains of corticosteroid hormone influences on brain activity: rapid, slow, and chronic modes. Pharmacological Reviews 64 (4), 901e938. https://doi.org/10. 1124/pr.112.005892. Juruena, M.F., Cleare, A.J., Papadopoulos, A.S., Poon, L., Lightman, S., Pariante, C.M., 2006. Different responses to dexamethasone and prednisolone in the same depressed patients. Psychopharmacology 189, 225e235. https:// doi.org/10.1007/s00213-006-0555-4. Juruena, M.F., Pariante, C.M., Papadopoulos, A.S., Poon, L., Lightman, S., Cleare, A.J., 2009. Prednisolone suppression test in depression: prospective study of the role of HPA axis dysfunction in treatment resistance. The British Journal of Psychiatry 194, 342e349. https://doi.org/10.1192/bjp.bp.108.050278. Kanatsou, S., Fearey, B.C., Kuil, L.E., Lucassen, P.J., Harris, A.P., Seckl, J.R., et al., 2015. Overexpression of mineralocorticoid receptors partially prevents chronic stress-induced reductions in hippocampal memory and structural plasticity. PLoS One 10 (11). https://doi.org/10.1371/journal.pone.0142012. Kanatsou, S., Karst, H., Kortesidou, D., van den Akker, R.A., den Blaauwen, J., Harris, A.P., et al., 2017. Overexpression of mineralocorticoid receptors in the mouse forebrain partly alleviates the effects of chronic early life stress on spatial memory, neurogenesis and synaptic function in the dentate gyrus. Frontiers in Cellular Neuroscience 11. https://doi.org/10.3389/fncel.2017.00132. Karssen, A.M., Meijer, O.C., Van Der Sandt, I.C.J., Lucassen, P.J., De Lange, E.C.M., De Boer, A.G., De Kloet, E.R., 2001. Multidrug resistance P-glycoprotein hampers the access of cortisol but not of corticosterone to mouse and human brain. Endocrinology. https://doi.org/10.1210/en.142.6.2686. Karst, H., Berger, S., Erdmann, G., Schütz, G., Joëls, M., 2010. Metaplasticity of amygdalar responses to the stress hormone corticosterone. Proceedings of the National Academy of Sciences of the United States of America 107 (32), 14449e14454. https://doi.org/10.1073/pnas.0914381107. Karst, H., Berger, S., Turiault, M., Tronche, F., Schutz, G., Joels, M., 2005. Mineralocorticoid receptors are indispensable for nongenomic modulation of hippocampal glutamate transmission by corticosterone. Proceedings of the National Academy of Sciences of the United States of America 102 (52), 19204e19207. https://doi.org/10. 1073/pnas.0507572102. Karst, H., Joëls, M., 2016. Severe stress hormone conditions cause an extended window of excitability in the mouse basolateral amygdala. Neuropharmacology 110, 175e180. https://doi.org/10.1016/j.neuropharm.2016.07.027. Kentrop, J., van der Tas, L., Loi, M., van IJzendoorn, M.H., Bakermans-Kranenburg, M.J., Joëls, M., van der Veen, R., 2016. Mifepristone treatment during early adolescence fails to restore maternal deprivation-induced deficits in behavioral inhibition of adult male rats. Frontiers in Behavioral Neuroscience 10. https://doi.org/10.3389/ fnbeh.2016.00122. Klok, M.D., Alt, S.R., Irurzun Lafitte, A.J.M., Turner, J.D., Lakke, E.A.J.F., Huitinga, I., et al., 2011a. Decreased expression of mineralocorticoid receptor mRNA and its splice variants in postmortem brain regions of patients with major depressive disorder. Journal of Psychiatric Research. https://doi.org/10.1016/j.jpsychires.2010.12.002. Klok, M.D., Giltay, E.J., Van Der Does, A.J.W., Geleijnse, J.M., Antypa, N., Penninx, B.W.J.H., et al., 2011b. A common and functional mineralocorticoid receptor haplotype enhances optimism and protects against depression in females. Translational Psychiatry. https://doi.org/10.1038/tp.2011.59. Klok, M.D., Vreeburg, S.A., Penninx, B.W.J.H., Zitman, F.G., de Kloet, E.R., DeRijk, R.H., 2011c. Common functional mineralocorticoid receptor polymorphisms modulate the cortisol awakening response: interaction with SSRIs. Psychoneuroendocrinology. https://doi.org/10.1016/j.psyneuen.2010.07.024.

References

59

Kuningas, M., de Rijk, R.H., Westendorp, R.G.J., Jolles, J., Slagboom, P.E., van Heemst, D., 2007. Mental performance in old age dependent on cortisol and genetic variance in the mineralocorticoid and glucocorticoid receptors. Neuropsychopharmacology 32 (6), 1295e1301. https://doi.org/10.1038/sj.npp.1301260. Lachize, S., Apostolakis, E.M., Van Der Laan, S., Tijssen, A.M.I., Xu, J., De Kloet, E.R., Meijer, O.C., 2009. Steroid receptor coactivator-1 is necessary for regulation of corticotropin-releasing hormone by chronic stress and glucocorticoids. Proceedings of the National Academy of Sciences of the United States of America. https://doi.org/10. 1073/pnas.0812062106. Lai, M., Horsburgh, K., Bae, S.E., Carter, R.N., Stenvers, D.J., Fowler, J.H., et al., 2007. Forebrain mineralocorticoid receptor overexpression enhances memory, reduces anxiety and attenuates neuronal loss in cerebral ischaemia. European Journal of Neuroscience 25 (6), 1832e1842. https://doi.org/10.1111/j.1460-9568.2007.05427.x. Lightman, S.L., Conway-Campbell, B.L., 2010. The crucial role of pulsatile activity of the HPA axis for continuous dynamic equilibration. Nature Reviews Neuroscience 11 (10), 710e718. https://doi.org/10.1038/nrn2914. Liu, W., Wang, J., Sauter, N.K., Pearce, D., 1995. Steroid receptor heterodimerization demonstrated in vitro and in vivo. Proceedings of the National Academy of Sciences of the United States of America 92 (26), 12480e12484. Retrieved from: http://www.pnas.org/cgi/reprint/92/26/12480. Loi, M., Sarabdjitsingh, R.A., Tsouli, A., Trinh, S., Arp, M., Krugers, H.J., Karst, H., van den Bos, R., Joëls, M., November 2, 2017a. Transient prepubertal mifepristone treatment normalizes deficits in contextual memory and neuronal activity of adult male rats exposed to maternal deprivation. eNeuro 4 (5). https://doi.org/ 10.1523/ENEURO.0253-17.2017. eCollection 2017 Sep-Oct pii: ENEURO.0253-17.2017. Loi, M., Mossink, J.C.L., Meerhoff, G.F., Den Blaauwen, J.L., Lucassen, P.J., Joëls, M., 2017b. Effects of early-life stress on cognitive function and hippocampal structure in female rodents. Neuroscience 342, 101e119. https://doi.org/ 10.1016/j.neuroscience.2015.08.024. Lopez, J.F., Chalmers, D.T., Little, K.Y., Watson, S.J., Lo, J.F., 1998. Mineralocorticoid receptor in rat and human Hippocampus : implications for the neurobiology of depression. Biological Psychiatry 3223 (97), 547e573. https:// doi.org/10.1016/S0006-3223(97)00484-8. McEwen, B.S., Bowles, N.P., Gray, J.D., Hill, M.N., Hunter, R.G., Karatsoreos, I.N., Nasca, C., 2015. Mechanisms of stress in the brain. Nature Neuroscience 18 (10), 1353e1363. https://doi.org/10.1038/nn.4086. Medina, A., Seasholtz, A.F., Sharma, V., Burke, S., Bunney, W., Myers, R.M., et al., 2013. Glucocorticoid and mineralocorticoid receptor expression in the human hippocampus in major depressive disorder. Journal of Psychiatric Research 47 (3), 307e314. https://doi.org/10.1016/j.jpsychires.2012.11.002. Mitra, R., Ferguson, D., Sapolsky, R.M., 2009. Mineralocorticoid receptor overexpression in basolateral amygdala reduces corticosterone secretion and anxiety. Biological Psychiatry 66 (7), 686e690. https://doi.org/10.1016/j. biopsych.2009.04.016. Muhtz, C., Zyriax, B.C., Bondy, B., Windler, E., Otte, C., 2011. Association of a common mineralocorticoid receptor gene polymorphism with salivary cortisol in healthy adults. Psychoneuroendocrinology 36, 298e301. https://doi. org/10.1016/j.psyneuen.2010.08.003. Murck, H., Schüssler, P., Steiger, A., 2012. Renin-angiotensin-aldosterone system: the forgotten stress hormone system: relationship to depression and sleep. Pharmacopsychiatry. https://doi.org/10.1055/s-0031-1291346. Oitzl, M.S., de Kloet, E.R., 1992 Feb. Selective corticosteroid antagonists modulate specific aspects of spatial orientation learning. Behavioral Neuroscience 106 (1), 62e71. Oitzl, M.S., Fluttert, M., Ron de Kloet, E., 1994. The effect of corticosterone on reactivity to spatial novelty is mediated by central mineralocorticosteroid receptors. European Journal of Neuroscience. https://doi.org/10.1111/j.14609568.1994.tb00604.x. Otte, C., Hinkelmann, K., Moritz, S., Yassouridis, A., Jahn, H., Wiedemann, K., Kellner, M., 2010. Modulation of the mineralocorticoid receptor as add-on treatment in depression: a randomized, double-blind, placebo-controlled proof-of-concept study. Journal of Psychiatric Research 44 (6), 339e346. https://doi.org/10.1016/j.jpsychires. 2009.10.006. Otte, C., Jahn, H., Yassouridis, A., Arlt, J., Stober, N., Maass, P., et al., 2003. The mineralocorticoid receptor agonist, fludrocortisone, inhibits pituitary-adrenal activity in humans after pre-treatment with metyrapone. Life Sciences 73, 1835e1845. https://doi.org/10.1016/S0024-3205(03)00513-7. Otte, C., Moritz, S., Yassouridis, A., Koop, M., Madrischewski, A.M., Wiedemann, K., Kellner, M., 2007. Blockade of the mineralocorticoid receptor in healthy men: effects on experimentally induced panic symptoms, stress hormones, and cognition. Neuropsychopharmacology 32. https://doi.org/10.1038/sj.npp.1301217.

60

4. The brain mineralocorticoid receptor: a resilience factor for psychopathology?

Otte, C., Wingenfeld, K., Kuehl, L.K., Kaczmarczyk, M., Richter, S., Quante, A., et al., 2015. Mineralocorticoid receptor stimulation improves cognitive function and decreases cortisol secretion in depressed patients and healthy individuals. Neuropsychopharmacology 40 (2), 386e393. https://doi.org/10.1038/npp.2014.181. Pariante, C.M., 2008. The role of multi-drug resistance p-glycoprotein in glucocorticoid function: studies in animals and relevance in humans. European Journal of Pharmacology. https://doi.org/10.1016/j.ejphar.2007.11.067. Qi, X.-R., Kamphuis, W., Wang, S., Wang, Q., Lucassen, P.J., Zhou, J.-N., Swaab, D.F., 2013. Aberrant stress hormone receptor balance in the human prefrontal cortex and hypothalamic paraventricular nucleus of depressed patients. Psychoneuroendocrinology 38, 863e870. https://doi.org/10.1016/j.psyneuen.2012.09.014. Rafatian, N., Westcott, K.V., White, R.A., Leenen, F.H.H., 2014. Cardiac macrophages and apoptosis after myocardial infarction: effects of central MR blockade. The Australian Journal of Pharmacy: Regulatory, Integrative and Comparative Physiology 307 (7), R879eR887. https://doi.org/10.1152/ajpregu.00075.2014. Ratka, A., Sutanto, W., BLoemers, M., De Kloet, E.R., 1989. On the role of brain mineralocorticoid (Type I) and glucocorticoid (Type II) receptors in neuroendocrine regulation. Neuroendocrinology. Retrieved from: http://www. scopus.com/inward/record.url?eid¼2-s2.0-0024361789&partnerID¼MN8TOARS. Reul, J.M.H.M., Collins, A., Saliba, R.S., Mifsud, K.R., Carter, S.D., Gutierrez-Mecinas, M., et al., 2015. Glucocorticoids, epigenetic control and stress resilience. Neurobiology of Stress 1 (1), 44e59. https://doi.org/10.1016/j. ynstr.2014.10.001. Reul, J.M.H.M., De Kloet, E.R., 1985. 2 receptor systems for corticosterone in rat-brain - microdistribution and differential occupation. Endocrinology 117 (6), 2505e2511. https://doi.org/10.1210/endo-117-6-2505. Reul, J.M., Labeur, M.S., Grigoriadis, D.E., De Souza, E.B., Holsboer, F., 1994. Hypothalamic-pituitary-adrenocortical axis changes in the rat after long-term treatment with the reversible monoamine oxidase-A inhibitor moclobemide. Neuroendocrinology 60, 509e519. https://doi.org/10.1159/000126788. Rozeboom, A.M., Akil, H., Seasholtz, A.F., 2007. Mineralocorticoid receptor overexpression in forebrain decreases anxiety-like behavior and alters the stress response in mice. Proceedings of the National Academy of Sciences of the United States of America 104 (11), 4688e4693. https://doi.org/10.1073/pnas.0606067104. Sapolsky, R.M., Romero, L.M., Munck, a. U., 2000. How do glucocorticoids influence stress responses ? Preparative actions. Endocrine Reviews 21 (April), 55e89. https://doi.org/10.1210/er.21.1.55. Schöbitz, B., Schöbitz, B., Sntanto, W., Carey, M.P., Holsboer, F., De Kloet, E.R., 1994. Endotoxin and interleukin 1 decrease the affinity of hippocampal mineralocorticoid (Type I) receptor in parallel to activation of the hypothalamic- pituitary-adrenal axis. Neuroendocrinology 60 (2), 124e133. https://doi.org/10.1159/000126742. Schwabe, L., Höffken, O., Tegenthoff, M., Wolf, O.T., 2013a. Stress-induced enhancement of response inhibition depends on mineralocorticoid receptor activation. Psychoneuroendocrinology 38 (10), 2319e2326. https://doi.org/ 10.1016/j.psyneuen.2013.05.001. Schwabe, L., Schächinger, H., de Kloet, E.R., Oitzl, M.S., 2010. Stress impairs spatial but not early stimulus-response learning. Behavioural Brain Research. https://doi.org/10.1016/j.bbr.2010.04.029. Schwabe, L., Tegenthoff, M., Höffken, O., Wolf, O.T., 2013b. Mineralocorticoid receptor blockade prevents stressinduced modulation of multiple memory systems in the human brain. Biological Psychiatry 74 (11), 801e808. https://doi.org/10.1016/j.biopsych.2013.06.001. Seckl, J.R., Fink, G., 1992. Antidepressants increase glucocorticoid and mineralocorticoid receptor mRNA expression in rat hippocampus in vivo. Neuroendocrinology 55, 621e626. Selye, H., 1950. STRESS - The Physiology and Pathology of Exposure to Stress. Acta Inc Montreal. https://doi.org/ 10.1016/S0016-5085(51)80143-4. Southwick, S.M., Bonanno, G.A., Masten, A.S., Panter-Brick, C., Yehuda, R., 2014. Resilience definitions, theory, and challenges: interdisciplinary perspectives. European Journal of Psychotraumatology 5. https://doi.org/10.3402/ ejpt.v5.25338. Spijker, A.T., Van Rossum, E.F.C., 2012. Glucocorticoid sensitivity in mood disorders. Neuroendocrinology. https:// doi.org/10.1159/000329846. ter Heegde, F., De Rijk, R.H., Vinkers, C.H., February 2015. The brain mineralocorticoid receptor and stress resilience. Psychoneuroendocrinology 52, 92e110. https://doi.org/10.1016/j.psyneuen.2014.10.022. Epub 2014 Nov 7. ter Horst, J.P., Kentrop, J., Arp, M., Hubens, C.J., de Kloet, E.R., Oitzl, M.S., 2013a. Spatial learning of female mice: a role of the mineralocorticoid receptor during stress and the estrous cycle. Frontiers in Behavioral Neuroscience 7 (May), 1e10. https://doi.org/10.3389/fnbeh.2013.00056.

References

61

ter Horst, J.P., Kentrop, J., de Kloet, E.R., Oitzl, M.S., 2013b. Stress and estrous cycle affect strategy but not performance of female C57BL/6J mice. Behavioural Brain Research 241 (1), 92e95. https://doi.org/10.1016/j.bbr. 2012.11.040. ter Horst, J.P., van der Mark, M., Kentrop, J., Arp, M., van der Veen, R., de Kloet, E.R., Oitzl, M.S., 2014. Deletion of the forebrain mineralocorticoid receptor impairs social discrimination and decision-making in male, but not in female mice. Frontiers in Behavioral Neuroscience. https://doi.org/10.3389/fnbeh.2014.00026. van Leeuwen, N., Bellingrath, S., de Kloet, E.R., Zitman, F.G., DeRijk, R.H., Kudielka, B.M., et al., 2011. Human mineralocorticoid receptor (MR) gene haplotypes modulate MR expression and transactivation: implication for the stress response. Psychoneuroendocrinology 36 (5), 699e709. https://doi.org/10.1016/j.psyneuen.2010.10.003. van Leeuwen, N., Caprio, M., Blaya, C., Fumeron, F., Sartorato, P., Ronconi, V., et al., 2010a. The functional c.-2G>C variant of the mineralocorticoid receptor modulates blood pressure, renin, and aldosterone levels. Hypertension. https://doi.org/10.1161/HYPERTENSIONAHA.110.155630. Van Weert, L.T.C.M., Buurstede, J.C., Mahfouz, A., Braakhuis, P.S.M., Polman, J.A.E., Sips, H.C.M., et al., 2017. NeuroD factors discriminate mineralocorticoid from glucocorticoid receptor DNA binding in the male rat brain. Endocrinology. https://doi.org/10.1210/en.2016-1422. Vinkers, C.H., Joels, M., Milaneschi, Y., Gerritsen, L., Kahn, R.S., Penninx, B.W.J.H., Boks, M.P.M., 2015. Mineralocorticoid receptor haplotypes sex-dependently moderate depression susceptibility following childhood maltreatment. Psychoneuroendocrinology 54, 90e102. https://doi.org/10.1016/j.psyneuen.2015.01.018. Vogel, S., Gerritsen, L., van Oostrom, I., Arias-Vasquez, A., Rijpkema, M., Joels, M., et al., 2014. Linking genetic variants of the mineralocorticoid receptor and negative memory bias: interaction with prior life adversity. Psychoneuroendocrinology 40, 181e190. https://doi.org/10.1016/j.psyneuen.2013.11.010. Vogel, S., Klumpers, F., Krugers, H.J., Fang, Z., Oplaat, K.T., Oitzl, M.S., et al., 2015. Blocking the mineralocorticoid receptor in humans prevents the stress-induced enhancement of centromedial amygdala connectivity with the dorsal striatum. Neuropsychopharmacology (40), 947e956. https://doi.org/10.1038/npp.2014.271. Vogel, S., Fernández, G., Joëls, M., Schwabe, L., March 2016. Cognitive adaptation under stress: a case for the mineralocorticoid receptor. Trends in Cognitive Sciences 20 (3), 192e203. https://doi.org/10.1016/j.tics.2015.12.003. Epub 2016 Jan 20. Review. Vogel, S., Klumpers, F., Schröder, T.N., Oplaat, K.T., Krugers, H.J., Oitzl, M.S., et al., 2017. Stress induces a shift towards striatum-dependent stimulus-response learning via the mineralocorticoid receptor. Neuropsychopharmacology 42 (6), 1262e1271. https://doi.org/10.1038/npp.2016.262. Wang, S.-S., Kamphuis, W., Huitinga, I., Zhou, J.-N., Swaab, D.F., 2008. Gene expression analysis in the human hypothalamus in depression by laser microdissection and real-time PCR: the presence of multiple receptor imbalances. Molecular Psychiatry 13, 786e799, 741. https://doi.org/10.1038/mp.2008.38. Weaver, I.C., Cervoni, N., Champagne, F.A., D’Alessio, A.C., Sharma, S., Seckl, J.R., et al., 2004. Epigenetic programming by maternal behavior. Nature Neuroscience 7 (8), 847e854. https://doi.org/10.1038/nn1276. Wellhoener, P., Born, J., Fehm, H.L., Dodt, C., 2004. Elevated resting and exercise-induced cortisol levels after mineralocorticoid receptor blockade with canrenoate in healthy humans. The Journal of Clinical Endocrinology and Metabolism 89. https://doi.org/10.1210/jc.2004-0086. Wirz, L., Reuter, M., Wacker, J., Felten, A., Schwabe, L., November 15, 2017. A haplotype Associated with enhanced mineralocorticoid receptor expression facilitates the stress-induced shift from “Cognitive” to “Habit” Learning. eNeuro 4 (6). https://doi.org/10.1523/ENEURO.0359-17.2017. eCollection 2017 Nov-Dec pii: ENEURO.035917.2017. Xing, G.-Q., Russell, S., Webster, M.J., Post, R.M., 2004. Decreased expression of mineralocorticoid receptor mRNA in the prefrontal cortex in schizophrenia and bipolar disorder. The International Journal of Neuropsychopharmacology 7, 143e153. https://doi.org/10.1017/S1461145703004000. Yau, J.L., Olsson, T., Morris, R.G., Meaney, M.J., Seckl, J.R., 1995 Jun. Glucocorticoids, hippocampal corticosteroid receptor gene expression and antidepressant treatment: relationship with spatial learning in young and aged rats. Neuroscience 66 (3), 571e581. Yau, J.L.W., Noble, J., Hibberd, C., Seckl, J.R., 2001. Short-term administration of fluoxetine and venlafaxine decreases corticosteroid receptor mRNA expression in the rat hippocampus. Neuroscience Letters 306 (3), 161e164. https:// doi.org/10.1016/S0304-3940(01)01890-0.

62

4. The brain mineralocorticoid receptor: a resilience factor for psychopathology?

Young, E.A., Lopez, J.F., Murphy-Weinberg, V., Watson, S.J., Akil, H., 1998. The role of mineralocorticoid receptors in hypothalamic-pituitary-adrenal axis regulation in humans. Journal of Clinical Endocrinology and Metabolism 83. https://doi.org/10.1210/jcem.83.9.5077. Zalachoras, I., Houtman, R., Meijer, O.C., 2013. Understanding stress-effects in the brain via transcriptional signal transduction pathways. Neuroscience 242, 97e109. Zhou, M., Kindt, M., Joëls, M., Krugers, H.J., 2011. Blocking mineralocorticoid receptors prior to retrieval reduces contextual fear memory in mice. PLoS One 6 (10). https://doi.org/10.1371/journal.pone.0026220.

C H A P T E R

5

GABAB receptors, depression, and stress resilience: a tale of two isoforms 1

Olivia F. O’Leary1, 2, John F. Cryan1, 2

Department of Anatomy and Neuroscience, University College Cork, Cork, Ireland; 2APC Microbiome Institute, University College Cork, Cork, Ireland

Introduction In the central nervous system, gamma-aminobutyric acid (GABA) acts on two types of receptors: ionotropic GABAA and GABAC receptors and metabotropic GABAB receptors. GABAA receptors are ligand-gated ion channels while GABAB receptors are G-protein-coupled receptors. GABAB receptors are found presynaptically where they function as either autoreceptors limiting the release of GABA or as heteroreceptors inhibiting the release of glutamate. However, these receptors are also found postsynaptically where they induce slow inhibitory postsynaptic currents (Cryan and Kaupmann, 2005; Bettler et al., 2004). Functional GABAB receptors are heterodimers of GABAB1 and GABAB2 subunits, and the GABAB1 subunit is expressed as several isoforms (Lee et al., 2010; Fritschy et al., 1999). The GABAB1a and GABAB1b isoforms are the predominant subunit isoforms that are expressed in the brain, whereby the GABAB1b subunit isoform is predominantly localized postsynaptically, whereas the GABAB1a subunit isoform is mainly found presynaptically (Fritschy et al., 1999; Gassmann and Bettler, 2012; Vigot et al., 2006). In dendrites, GABAB1a localizes to glutamatergic terminals for heteroreceptor function, whereas GABAB1b localizes to spines opposing glutamate release sites, thus affecting presynaptic or postsynaptic inhibition (Gassmann and Bettler, 2012). Structurally, these two isoforms differ only by the presence of a sushi domain in the N-terminus of the GABAB1a receptor subunit isoform, which is thought to increase surface stability of GABAB(1a, 2) receptors and promote their axonal localization (Gassmann and Bettler, 2012; Hannan et al., 2012). Interest in the role of the GABAB receptor in stress resilience began 30 years ago, when a potential role for the GABAB receptor in the pathophysiology and treatment of the Stress Resilience https://doi.org/10.1016/B978-0-12-813983-7.00005-7

63

Copyright © 2020 Elsevier Inc. All rights reserved.

64

5. GABAB receptors, depression, and stress resilience: a tale of two isoforms

stress-related disorder, depression, was first reported (Pilc and Lloyd, 1984). Since then, many preclinical studies have reported a reciprocal but complex relationship between stress-related psychiatric disorders such as depression and anxiety with the GABAB receptor as outlined in the following sections.

The impact of stress-related psychiatric disorders and their treatments on GABAB receptor density, gene expression and function Effects of antidepressants on GABAB receptor density in rodents The first hint that the GABAB receptor may play a role in stress-related responses came from receptor binding studies in the early 1980s, which examined the effects of antidepressant treatments on GABAB receptor density (Pilc and Lloyd, 1984). Since then, many other studies have reported that this receptor is affected by several different antidepressant treatments although some conflicting and brain region-dependent results have also been reported (Felice et al., 2016; Ghose et al., 2011; Cryan and Slattery, 2010; Enna and Bowery, 2004). It was reported that chronic but not acute treatment with the antidepressants amitriptyline, desipramine, or citalopram increased GABAB receptor-binding sites (i.e., receptor density) in the rat frontal cortex (Pilc and Lloyd, 1984). These findings were reproduced in a later study, which also reported that other antidepressant treatments including fluoxetine, mianserin, trazodone, and repeated electroconvulsive shocks upregulated GABAB receptor binding in the rat frontal cortex (Lloyd et al., 1985). Such effects of antidepressants on GABAB receptor binding seemed to be region dependent and were not apparent in the rat hippocampus (Lloyd et al., 1985). Chronic treatment with the antidepressant imipramine was also shown to increase GABAB receptor binding in the mouse cortex (Suzdak and Gianutsos, 1986). However, opposing data have also been reported. For instance, Pratt and Bowery (1993) reported that although desipramine increased GABAB receptor binding in the rat frontal cortex, neither paroxetine nor amitriptyline had any effect (Pratt and Bowery, 1993). Similarly, a lack of effect of desipramine, imipramine, and tranylcypromine on frontal cortex GABAB receptor binding has been reported by others (Cross and Horton, 1987, 1988; McManus and Greenshaw, 1991). On the other hand, the antidepressants desipramine and imipramine have been shown to reverse the reduction in frontal cortex GABAB receptor that is induced by learned helplessness, thus suggesting that antidepressants may affect GABAB receptor density not only under basal conditions but also in depression-like states (Martin et al., 1989).

Effects of antidepressants on GABAB receptor function in rodents These findings of antidepressant-induced increases in GABAB receptor density in the cortex are consistent with findings that GABAB receptor function is increased following various antidepressant treatments. Indeed, it has been reported that imipramine-induced increases in GABAB receptor density were accompanied by enhanced receptor function as shown by the potentiation of baclofen (a GABAB receptor agonist)-induced adenylate cyclase activity in the mouse cerebral cortex (Suzdak and Gianutsos, 1986).

The impact of stress-related psychiatric disorders and their treatments on GABAB receptor density, gene expression and function

65

Repeated treatment with the antidepressant tranylcypromine has also been reported to enhance GABAB receptor function in the rat cerebral cortex as measured by baclofenstimulated GTPgS binding (Sands et al., 2003). Similarly, chronic treatment with amitriptyline, desipramine, mianserin, or electroconvulsive shock increased GABAB receptoremediated modulation of serotonin release in the mouse frontal cortex (Gray and Green, 1987). In contrast to all of these findings, however, one study reported that antidepressants including desipramine and imipramine do not alter GABAB receptor function in the cerebral cortex (Szekely et al., 1987). Interestingly, 7 days treatment with the antidepressants tranylcypromine, phenelzine, or desipramine (but not fluoxetine) increased GABAB receptor function in the rat hippocampus (Sands et al., 2004). This suggests that although antidepressants affect GABAB receptor density in the frontal cortex but not hippocampus (Lloyd et al., 1985), these drugs can affect receptor function in both the hippocampus and the frontal cortex (Lloyd et al., 1985; Sands et al., 2004). However, it should also be noted that the direction of change of antidepressant effects on GABAB receptor function may be region dependent, as it has been reported that chronic fluoxetine treatment reduced GABAB receptoreinduced GIRK responses in the rat dorsal raphe nucleus (DRN) (Cornelisse et al., 2007).

Clinical evidence of altered GABAB receptor density and function in depression and the antidepressant response All of this preclinical evidence is supported by a growing body of clinical evidence of GABAB receptor dysfunction in depression (for review, see Felice et al., 2016; Ghose et al., 2011). Indeed, recent clinical neurophysiology studies suggest that deficits in GABAB receptors may play a role in major depression and the antidepressant response to fluoxetine (Levinson et al., 2010; Croarkin et al., 2014). In addition, the induction of growth hormone release by the GABAB receptor agonist baclofen is blunted in depressed patients (O’Flynn and Dinan, 1993; Marchesi et al., 1991). Moreover, it has been reported that the GABAB2 receptor subunit is upregulated in cortical and subcortical brain regions in depressed suicide victims compared with those without a history of depression (Klempan et al., 2009). Similarly, a 50% increase in GABAB2 subunit gene expression was reported in the dentate gyrus of the hippocampus in depressed individuals (Ghose et al., 2011). However, this upregulation is not reflected in earlier receptor binding studies in which similar GABAB receptorebinding profiles in the frontal or temporal cortices and the hippocampus had been reported in depressed suicide victims and controls (Cross et al., 1988) and in the frontal cortex of suicide victims and controls (Arranz et al., 1992).

Alterations in GABAB receptor density and function in animal models of stress and depression Considering the relatively strong evidence for a potential role of the GABAB receptor in depression and the response to antidepressants, it is somewhat surprising that only a limited number of preclinical studies have measured GABAB receptor expression and

66

5. GABAB receptors, depression, and stress resilience: a tale of two isoforms

function in animal models of depression and stress. It has been reported that GABAB receptor binding in the frontal cortex is reduced in the rat learned helplessness model of depression (Martin et al., 1989). On the other hand, however, chronic restraint stress (7 days) had no effect on GABAB receptor activity in the cerebral cortex as measured by baclofen-stimulated GTPgS binding (Sands et al., 2003). Similarly, social stress did not affect GABAB receptoremediated GIRK currents in the rat DRN (Cornelisse et al., 2007). In summary, most preclinical studies have reported that chronic treatment with several different antidepressants increases GABAB receptor density and activity particularly in the cortex. In support, clinical studies suggest that deficits in GABAB receptor function may play a role in major depression and the antidepressant response to fluoxetine; however, only a limited number of clinical studies have actually been conducted. Although no differences in GABAB receptorebinding profiles have been reported in the frontal and temporal cortices of the suicide brain, increased expression of the GABAB2 receptor subunit has been reported in the hippocampus and cortex of depressed suicide victims. Taken together, these preclinical and clinical studies on GABAB receptor density and activity suggest a role for the GABAB receptor in antidepressant action and perhaps a more indirect or limited role in the pathophysiology of stress and depression. Nevertheless, as outlined in the following sections, it is clear that altering GABAB receptor activity regulates depression and anxietylike behaviors and behavioral responses to stress.

Effects of GABAB receptor modulation on depression-like behaviors There is a large body of convincing preclinical evidence that pharmacological or genetic manipulation of the GABAB receptor affects anxiety-, depression-, and antidepressantrelated behaviors (Cryan and Kaupmann, 2005; Felice et al., 2016; Cryan and Slattery, 2010). Mice with null mutations of either the GABAB1 or GABAB2 receptor subunits demonstrate an antidepressant-like behavioral phenotype as indicated by reduced immobility in the forced swim test (FST; Mombereau et al., 2004, 2005). Moreover, GABAB receptor antagonists exert antidepressant-like effects in the FST, and in the learned helplessness, olfactory bulbectomy, and chronic mild stress paradigms (reviewed in Felice et al., (2016)). Specifically, the GABAB receptor antagonists CGP56433A, CGP51176, CGP5633A, CGP36742, and SCH50911 have all been shown to induce antidepressant-like behavior in the FST in rats or mice (Mombereau et al., 2004; Slattery et al., 2005; Nowak et al., 2006; Frankowska et al., 2007; Felice et al., 2012). GABAB receptor antagonists are also effective in other rodent models of depression and antidepressant activity including the olfactory bulbectomy model (CGP36742 and CGP51176) (Nowak et al., 2006), chronic mild stress (CGP51176) (Nowak et al., 2006), and learned helplessness model (CGP36742) (Nakagawa et al., 1999). On the other hand, it is positive allosteric modulators (PAMs) of GABAB receptors that exert anxiolytic effects in tests of innate anxiety although PAMs do not exert any effects in conditioned fear paradigms (Frankowska et al., 2007; Sweeney et al., 2013; Li et al., 2015; Cryan et al., 2004). Taken together, it is clear that reducing GABAB receptor function has antidepressant-like effects, whereas positive allosteric modulation of the receptor has anxiolytic effects.

The role of GABAB1 receptor subunit isoforms in stress resilience

67

The role of GABAB1 receptor subunit isoforms in stress resilience Although it is clear that pharmacological modulation of GABAB receptors can influence depression and anxiety-like behaviors, relatively few studies have examined their effects in the behavioral responses to chronic stress. Nevertheless, it has been reported that the GABAB receptor antagonist, CGP51176, prevented anhedonia in the chronic mild stress paradigm (Nowak et al., 2006). More recently, we have used GABABð1aÞ-=- and GABABð1bÞ-=- mice as tools to delineate the roles of specific GABAB1 receptor subunit isoforms in depression and anxiety-related behaviors as well as in resilience and susceptibility to stress-induced changes in these behaviors. Indeed, we recently reported that GABAB1a and GABAB1b receptor subunit isoforms differentially regulate stress resilience in both male and female mice (O’Leary et al., 2014) (Fig. 5.1). Specifically, male GABABð1aÞ-=- mice were more susceptible, whereas male GABABð1bÞ-=- mice were more resilient to chronic social defeat stress (O’Leary et al., 2014). In the social interaction test, social defeat stress decreased social interaction to a greater extent in GABABð1aÞ-=- mice when compared with wild-type mice. On the other hand, mice lacking the GABABð1bÞ-=- isoform were resilient to this stress-induced social avoidance. Moreover,

FIGURE 5.1 Summary of the main differences between GABABð1aÞ-=- mice in their response to stress and potential underlying mechanisms of stress resilience in GABABð1bÞ-=- mice. CORT, corticosterone; DRN, dorsal raphe nucleus; FST, forced swim test; MS, maternal separation; NAcc, nucleus accumbens; SDS, social defeat stress; TST, tail suspension test.

68

5. GABAB receptors, depression, and stress resilience: a tale of two isoforms

social defeat stress induced anhedonia in GABABð1aÞ-=- mice as measured by reduced preference to drink a sweet solution of saccharin over water, whereas GABABð1bÞ-=- mice were resilient to this measure of stress-induced anhedonia. In an independent cohort of mice, we also examined the impact of early-life stress on depression and anxiety-related behaviors in adulthood and using a model amenable to both male and female mice (O’Leary et al., 2014). To this end, we used unpredictable maternal separation combined with unpredictable maternal stress, whereby pups were separated from their mother at an unpredictable time during the light or dark cycle and during which time the mother was also exposed to a brief unpredictable stressor at an unpredictable time during the 3 h separation period. In agreement with our findings in socially defeated male GABABð1aÞ-=- mice, we found that maternally separated female GABABð1bÞ-=- mice also exhibited an anhedonic-like response in the saccharin preference test when compared with both GABABð1bÞ-=- and wild-type mice. We also assessed the effects of maternal separation on anhedonia in male mice using the female urine sniffing test (Malkesman et al., 2010). In this test, a cotton bud dipped in water or the urine of female mice that are in estrus is presented to the male mouse in its home cage. Male mice tend to spend more time sniffing the urine over water, and this is taken as a measure of sexual interest. It has been previously shown that rodents that exhibit learned helplessness spend less time sniffing the urine, an effect prevented by chronic antidepressant treatment (Malkesman et al., 2010). Maternal separation decreased the preference for urine in wild-type mice and completely abolished this preference in GABABð1aÞ-=- mice while having no effect in GABABð1bÞ-=- mice (O’Leary et al., 2014). Together with the findings from the social defeat experiment, this suggests that GABABð1aÞ-=- mice are more susceptible, whereas GABABð1bÞ-=- mice are more resilient to stress-induced anhedonia. The impact of chronic stress on antidepressant-like behavior was also assessed in these mice using the FST and tail suspension test, but the findings from these tests were more complex than those assessing anhedonia readouts. This was due in part to genotype differences under control conditions. Under baseline conditions, both GABABð1aÞ-=- and GABABð1bÞ-=- mice exhibited decreased immobility in the FST (O’Leary et al., 2014). Although these findings might be interpreted as both GABABð1aÞ-=- and GABABð1bÞ-=- mice having an antidepressant-like phenotype in the FST, a different picture emerges when they were exposed to maternal separation stress. In the FST we found that female (but not male) maternally separated GABABð1bÞ-=- mice retained their antidepressant-like behavioral phenotype suggesting they were more stress resilient, whereas maternally separated GABABð1aÞ-=- mice did not retain this phenotype. In addition, in the tail suspension test, both nonstressed and stressed male and female GABABð1aÞ-=- mice exhibited increased immobility, thus suggesting that GABABð1aÞ-=- mice have a depression-like behavioral phenotype in this test. On the other hand, both nonstressed and stressed male GABABð1bÞ-=- mice exhibited reduced immobility, thus suggesting an antidepressant-like phenotype. However, caution is required when interpreting these reductions in immobility in GABABð1bÞ-=- mice, as another study did not observe this decreased immobility in the FST (Jacobson et al., 2017), and these mice also show increased locomotor activity (O’Leary et al., 2014). Nevertheless, we also observed the female maternally separated GABABð1bÞ-=- mice do not exhibit hyperactivity and yet exhibit reduced immobility in the FST; thus a role in antidepressant-like behavior cannot be completely discounted.

Potential mechanisms underlying the differential roles of GABAB1a and GABAB1b receptor subunit isoforms in stress resilience

69

Although the data gathered from GABABð1aÞ-=- and GABABð1bÞ-=- mice suggest that deletion of either subunit isoform can differentially modulate stress resilience, surprisingly little work has been done on investigating whether the expression of these subunits is altered by stress or models of stress-related psychiatric disorders. Nevertheless, we have reported that the helpless H/Rouen genetic mouse model of depression exhibits increased GABAB(1b) mRNA expression in the hippocampus when compared with their nonhelpless controls (O’Leary et al., 2014) and that a probiotic that promotes antidepressant-like effects increased the expression of this subunit in the mouse hippocampus (Bravo et al., 2011). In contrast, we did not observe any changes in hippocampal GABAB1a mRNA expression in the helpless H/Rouen mouse strain (O’Leary et al., 2014). On the other hand, a previous small postmortem human brain study reported that GABAB1a mRNA expression was decreased in the dentate gyrus of depressed individuals (Ghose et al., 2011), and others have reported antidepressant-induced increases in GABAB1a mRNA expression in the rat hippocampus (Sands et al., 2004). Given these somewhat opposing findings, a more systematic investigation of brain-wide impact of depression, stress, and antidepressant treatments on the expression of these subunit isoforms is required. Taken together, these studies support a role for the GABAB1a receptor in depression, in antidepressant action, and in determining stress susceptibility, whereby reduced GABAB1a expression in the hippocampus is associated with depression in humans and increased stress susceptibility in mice, whereas increased hippocampal expression seems to occur following antidepressant treatment. On the other hand, increased GABAB1b expression in the hippocampus is a phenotype of a genetic mouse model of depression, and deletion of this subunit increases stress resilience in mice.

Potential mechanisms underlying the differential roles of GABAB1a and GABAB1b receptor subunit isoforms in stress resilience The neurobiological mechanisms underlying the differential roles of the GABAB1a and GABAB1b receptor subunit isoforms in stress resilience are not yet fully understood, but as outlined in the following sections, several studies suggest that the serotonin neurotransmitter system, the hypothalamic-pituitary-adrenal (HPA) axis, selected brain regions, and adult hippocampal neurogenesis may play a role.

The serotonin neurotransmitter system The serotonin (5-HT) neurotransmitter system has long been implicated in the pathophysiology and treatment of the stress-related psychiatric disorder depression (O’Leary and Cryan, 2010; Lucki, 1998). Indeed, the selective serotonin reuptake inhibitor (SSRI) antidepressants were developed to increase synaptic availability of this neurotransmitter in the brain. There is growing evidence of a functional link between the serotonin system and GABAB receptors. Indeed, most 5-HT cell bodies in the dorsal and medial raphe nuclei express the GABAB receptor (Abellan et al., 2000a; Varga et al., 2002). In addition, it has been demonstrated that activation of GABAB receptors by the agonist, baclofen, modulates the release of 5-HT in the DRN, the nucleus accumbens, and the striatum (Abellan et al.,

70

5. GABAB receptors, depression, and stress resilience: a tale of two isoforms

2000a,b; Tao et al., 1996; Takahashi et al., 2010). Moreover, we have previously shown that the antidepressant-like effects of GABAB receptor antagonists in the rat-FST are dependent on an intact serotonergic system (Slattery et al., 2005). Reciprocally, mice lacking the serotonin transporter exhibit desensitized GABAB receptors in the raphe nuclei (la Cour et al., 2004), and rats chronically treated with the SSRI, fluoxetine, exhibit reduced GABAB receptoremediated GIRK responses in the DRN (Cornelisse et al., 2007). Importantly in the context of stress, it has been shown that social defeat stress increases GABA-mediated inhibition of 5-HT in the DRN of stress-susceptible mice, whereas GABA silencing disinhibited serotonergic cells and promoted a stress-resilient phenotype in mice exposed to social defeat (Challis et al., 2013). This suggests that GABA-serotonin interactions in the DRN play a role in stress resilience. Indeed, we have found that GABABð1bÞ-=- mice (which are more stress resilient) exhibit enhanced stress-induced expression of the immediate early gene, c-Fos, in the DRN, when compared with the stresssusceptible GABABð1aÞ-=- or the normo-stress-sensitive wild-type mice, thus suggesting that the DRN is a key brain region involved in GABAB1 receptor subunit regulation of stress resilience (O’Leary et al., 2014). The 5-HT1A receptor is thought to play a pivotal role in the stress-related psychiatric disorders, depression, and anxiety (O’Leary and Cryan, 2010; Blier and Ward, 2003; Cryan and Leonard, 2000). These receptors are localized in the raphe nuclei where they act as somatodendritic autoreceptors that inhibit 5-HT cell firing but are also found postsynaptically in a number of limbic brain regions important in the regulation of emotion, such as the hippocampus (Hoyer et al., 2002). Desensitization of this receptor has been long implicated in the mechanism of antidepressant action (Blier and Ward, 2003; Albert et al., 2014; Hensler, 2003; De Vry, 1995) and in enhancing the onset of antidepressant action perhaps through increased 5-HT availability in the forebrain (Artigas et al., 1996; Blier et al., 1997; Ferres-Coy et al., 2013). Thus, we recently examined 5-HT1a receptoremediated responses in both GABABð1aÞ-=- and GABABð1bÞ-=- mice (Jacobson et al., 2017). In this study, both male and female GABABð1aÞ-=- mice exhibited a blunted hypothermic response to the 5-HT1A receptor agonist 8-OH-DPAT, thus suggesting that these mice have impaired presynaptic 5-HT1A autoreceptor function (Jacobson et al., 2017). In agreement with these findings, previous in situ hybridization studies suggest that it is the GABAB1a isoform that is predominantly expressed on serotonergic cell bodies in the DRN (Bischoff et al., 1999). GABABð1aÞ-=- mice also exhibited attenuated 8-OH-DPAT-induced stimulation of the HPA axis and body posture flattening, thus suggesting that postsynaptic 5-HT1A receptors are also densensitized, although this desensitization seems to be weaker than that occurring at presynaptic 5-HT1A receptors (Jacobson et al., 2017). These effects were generally not associated with alterations in 5-HT1A receptor expression nor with alterations in 5-HT1a receptor G-protein coupling. In addition, no alterations in 8-OH-DPAT-induced responses were observed in the GABABð1bÞ-=- mice (Jacobson et al., 2017). Taken together, these data suggest that the DRN may be an important brain region involved in the stress-resilient phenotype of GABABð1bÞ-=- mice and that the sensitivity of presynaptic and postsynaptic 5-HT1A receptors is reduced in the stress-susceptible GABABð1aÞ-=- mice. Given the role of 5-HT1A receptor desensitization in the response to SSRI antidepressants, it will be of interest to determine whether GABABð1aÞ-=- mice exhibit altered sensitivity to these drugs.

Potential mechanisms underlying the differential roles of GABAB1a and GABAB1b receptor subunit isoforms in stress resilience

71

The hypothalamic-pituitary-adrenal axis Because the HPA axis is strongly implicated in stress resilience and vulnerability (Franklin et al., 2012; Reul et al., 2015; Henckens et al., 2016), it comes as no surprise that its function has been somewhat interrogated in GABAB1 subunit isoform knockout mice (O’Leary et al., 2014; Jacobson et al., 2017). Under baseline conditions, plasma corticosterone concentrations do not seem to differ between wild-type, GABABð1aÞ-=- , and GABABð1bÞ-=- male mice (Jacobson et al., 2017). However, GABABð1aÞ-=- mice exhibit blunted corticosterone and adrenocorticotropic hormone (ACTH) release in response to the 5-HT1A receptor agonist 8-OH-DPAT, whereas GABABð1bÞ-=- mice did not differ from wild-type mice (Jacobson et al., 2017). This suggests that serotonergic regulation of the HPA axis is impaired in GABABð1aÞ-=- mice. We have also examined the impact of stress on plasma corticosterone in GABABð1bÞ-=- and GABABð1bÞ-=- mice. In male mice, stress-induced increases in plasma corticosterone were decreased in GABABð1aÞ-=- mice and increased in GABABð1bÞ-=mice (O’Leary et al., 2014). However, these differences in stress-induced corticosterone concentrations cannot fully explain the differential susceptibility of GABABð1aÞ-=- and GABABð1bÞ-=- mice to stress-induced changes in behavior. This is because female GABABð1aÞ-=- and GABABð1bÞ-=- mice did not differ in their corticosterone response to stress but yet exhibited differential susceptibility to stress-induced changes in depression-like behavior (O’Leary et al., 2014). However, it must also be kept in mind that there is sexual dimorphism in HPA axis regulation, and thus, it is unlikely that stress-induced changes in this system and its contribution to stress resilience would be similar in both males and females (Goel et al., 2014; Bangasser and Valentino, 2012).

Location, location, location. Toward identifying the neural circuitry underlying the differential stress susceptibility between GABABð1aÞ-=- and GABABð1bÞ-=- mice, we measured the effects of acute restraint stress on the expression of c-Fos (an immediate early gene) in several stress-related brain areas in adult wild-type, GABABð1aÞ-=-, and GABABð1bÞ-=- mice, with and without prior maternal separation stress. The nucleus accumbens was the only area where stress-induced c-Fos expression was differentially regulated in the stress-resilient GABABð1bÞ-=- mice by prior exposure to maternal separation stress. Specifically, maternal separation significantly increased stressinduced c-Fos expression in the nucleus accumbens of GABABð1bÞ-=- mice but not in wildtype or GABABð1aÞ-=- mice, and these effects of acute stress were not apparent in GABABð1bÞ-=- mice that had not undergone prior maternal separation. This suggests that the nucleus accumbens may be a key node in the neural circuitry of stress resilience in GABABð1bÞ-=- mice. Interestingly, GABABð1aÞ-=- mice exhibited decreased stress-induced cFos activation in the ventral tegmental area (VTA), and this effect was not apparent in GABABð1aÞ-=- mice that had undergone maternal separation, thus suggesting that the VTA might play a role in the stress-susceptible phenotype of these mice. Indeed, dysfunction of the nucleus accumbens and its associated reward circuitry including the VTA has already been implicated in susceptibility to stress-induced anhedonia (Russo and Nestler, 2013). Moreover, it is well established that GABAB receptors modulate VTA and nucleus

72

5. GABAB receptors, depression, and stress resilience: a tale of two isoforms

accumbensemediated hedonic processing as systemic, intra-VTA, or intranucleus accumbens (shell) administration of GABAB receptor agonists attenuates the rewarding effects of several drugs of abuse (Vlachou and Markou, 2010). Although there is clear role for GABAB receptor modulation of the hedonic effects of drugs of abuse, relatively little is known about GABAB receptor modulation of stress-induced anhedonia. Nevertheless, one study reported that reductions in sucrose preference induced by chronic mild stress in rats were prevented by chronic treatment with a GABAB receptor antagonist (Nowak et al., 2006), and we have shown that GABABð1aÞ-=- mice are more susceptible, whereas GABABð1bÞ-=- mice are more resilient to stress-induced anhedonia (O’Leary et al., 2014). Taken together, stress-induced differential neuronal activity patterns in the VTAnucleus accumbens reward system of the brain in GABABð1bÞ-=- and GABABð1aÞ-=- mice may at least partially contribute their differential susceptibility to stress-induced anhedonia although this has yet to be directly tested by inhibiting these isoforms specifically in the nucleus accumbens or VTA. One of the most robust genotype-dependent effects of acute stress on c-Fos expression was observed in the hippocampus (O’Leary et al., 2014), a key brain area involved in regulation of the stress response (Jacobson and Sapolsky, 1991; Brown et al., 1999) whereby the number of cFos-positive cells in response to acute stress was significantly increased in GABABð1bÞ-=- mice compared with wild-type and GABABð1aÞ-=- mice. This enhanced stress-induced c-Fos activation was most apparent in the dentate gyrus and ventral CA3 regions of the hippocampus and occurred to the same extent in both nonseparated and maternally separated GABABð1bÞ-=- mice. Interestingly, GABABð1aÞ-=- mice exhibited decreased stress-induced c-Fos in the ventral CA3, and this effect was not apparent when they had undergone prior maternal separation. Together, this suggests that GABAB receptors in the hippocampus might be important in the differential response to stress, and this observation is further supported by our findings (described in the next section) that maternal separation stress differentially affects adult hippocampal neurogenesis in GABABð1bÞ-=- versus GABABð1aÞ-=- mice. Finally, we also observed that GABABð1bÞ-=- mice exhibited enhanced stress-induced neural activation in the DRN irrespective of whether they had undergone prior maternal separation or not, but unlike the hippocampus, these increases were largely restricted to comparisons with GABABð1aÞ-=- and not wild-type mice (O’Leary et al., 2014). As described earlier in this chapter, there is accumulating evidence of a GABAB-serotonin interactions in the DRN (Cornelisse et al., 2007; Slattery et al., 2005; Takahashi et al., 2010), and the DRN plays a role in stress resilience (Challis et al., 2013). Taken together, this supports the hypothesis that GABAB receptors in the DRN may play a role in stress resilience. Within this context, however, it is noteworthy that it is the GABAB1a isoform rather than the GABAB1b isoform that is mainly expressed on serotonergic cells bodies DRN (Bischoff et al., 1999) and yet we did not observe any differences in stress-induced c-FOS expression in GABABð1aÞ-=- mice when compared with wild-type mice.

Adult hippocampal neurogenesis: a mechanism for resilience? Neurogenesis, the birth of new neurons, occurs in just a few areas of the adult brain including the dentate gyrus of the hippocampus, and several extrinsic factors can alter the

Potential mechanisms underlying the differential roles of GABAB1a and GABAB1b receptor subunit isoforms in stress resilience

73

proliferation and survival of these adult-born neurons including stress and chronic antidepressant treatments (Kempermann et al., 2015; Bergmann et al., 2015; Christian et al., 2014; Malberg et al., 2000; O’Leary et al., 2013). Moreover, there is emerging evidence that these adult-born hippocampal neurons may play a role in buffering the stress response as well as antidepressant regulation of the HPA axis (Levone et al., 2015; Snyder et al., 2011; Surget et al., 2011). Accumulating evidence suggests that the GABAB receptor also modulates adult hippocampal neurogenesis (Felice et al., 2012; O’Leary et al., 2014; Giachino et al., 2014). GABAB1-=- mice exhibit increased adult hippocampal progenitor cell proliferation as well as accelerated neuronal differentiation when compared with their wild-type counterparts (Giachino et al., 2014). In addition, we have shown that a GABAB receptor antagonist, CGP52432, which has antidepressant-like behavioral effects in the FST increased hippocampal cell proliferation in mice (Felice et al., 2012). Interestingly, we observed that the effect of CGP52432 on hippocampal cell proliferation occurred in the ventral rather than the dorsal hippocampus. This finding is intriguing in light of a growing body of evidence that suggests the hippocampus is functionally segregated along its longitudinal axis into dorsal and ventral regions, whereby the dorsal hippocampus (dHi) plays a predominant role in the spatial learning and memory, whereas the ventral hippocampus (vHi) plays a predominant role in the regulation of emotion-related processes (Bannerman et al., 2004; Fanselow and Dong, 2010). Moreover, there is emerging evidence that adult neurogenesis may also be differentially regulated along this axis with the effects of stress on neurogenesis occurring predominantly in the vHi (Tanti and Belzung, 2013; O’Leary and Cryan, 2014; O’Leary et al., 2012). Given the impact of stress and the GABAB receptor on adult hippocampal neurogenesis, and the identification of the hippocampus as a key brain area expressing altered neuronal responses to stress (as measured by c-Fos expression) in GABABð1bÞ-=- versus GABABð1aÞ-=mice, we examined whether increased adult hippocampal neurogenesis may contribute to the stress-resilient phenotype of the GABABð1bÞ-=- mice (O’Leary et al., 2014). We found that male GABABð1bÞ-=- mice exhibit increased proliferation of newly born cells in the vHi but not dHi. We also found that these stress-resilient GABABð1bÞ-=- mice exhibited increased survival of new adult-born cells in the hippocampus. Interestingly, we found that under baseline conditions, this increase in the survival of new adult-born cells occurred in the dHi but not the vHi, but in GABABð1bÞ-=- mice that had undergone early-life stress (maternal separation), this increased survival of new adult-born cells shifted from the dorsal to the ventral hippocampus. Thus, further supporting the emerging view that adult neurogenesis in the vHi rather than the dHi plays a predominant role in the response to stress. GABABð1bÞ-=- mice were also resistant to the early-life stress-induced decrease in the survival of adult-born cells in the vHi, thus suggesting a possible mechanism underlying their stress-resilient behavioral phenotype. Using female mice, we confirmed that increased adult hippocampal neurogenesis occurs in GABABð1bÞ-=- mice, whereas no differences were observed between GABABð1aÞ-=mice and wild-type mice. This finding also suggests that the increases in adult hippocampal neurogenesis observed in GABABð1bÞ-=- mice is not sex dependent and thus parallels the sex-independent stress-resilient behavioral phenotype observed in these mice (O’Leary et al., 2014).

74

5. GABAB receptors, depression, and stress resilience: a tale of two isoforms

The mechanisms underlying GABAB receptor modulation of adult hippocampal neurogenesis are not yet known but may involve regulation of brain-derived neurotrophic factor (BDNF). BDNF is a neurotrophin involved in adult hippocampal neurogenesis. Moreover, it is required for antidepressant-induced increases in adult hippocampal neurogenesis and antidepressant-like behavior, and it also plays a role in stress resilience (O’Leary and Castren, 2010; Sairanen et al., 2005; Saarelainen et al., 2003; Berton et al., 2006; Krishnan et al., 2007; Bjorkholm and Monteggia, 2016). GABAB receptor antagonists have been shown to elevate BDNF protein and mRNA levels in various brain regions including the hippocampus (Heese et al., 2000; Enna et al., 2006). Thus, it will be of interest to determine whether the stress-resilient phenotypes of GABABð1bÞ-=- are due to enhanced BDNF signaling.

Conclusions In summary, GABAB1 receptor subunit isoforms differentially regulate stress resilience (Fig. 5.1). Reductions or deletions of GABAB1b are associated with an antidepressant-like behavioral phenotype and resilience to psychostress-induced anhedonia and psychosocial stress-induced social avoidance, whereas increased hippocampal GABAB1b expression in the hippocampus has been found in a genetic mouse model of depression. On the other hand, mice lacking the GABAB1a receptor subunit isoform are more susceptible to stressinduced anhedonia and social avoidance. Experiments using c-Fos immunohistochemistry to delineate the neural circuitry underlying the differential stress sensitivity of GABABð1bÞ-=and GABABð1aÞ-=- mice suggest that the VTA-nucleus accumbens reward pathway, the DRN, and the hippocampus are likely key brain areas involved in this neural circuitry. Moreover, adult hippocampal neurogenesis and the serotonin neurotransmitter system have been shown to be differentially affected in GABABð1aÞ-=- and GABABð1bÞ-=- mice. Taken together, the GABAB1a and GABAB1b subunit isoforms represent potential novel therapeutic targets for the treatment of stress-related psychiatric disorders.

Acknowledgements We thank Dr. Daniela Felice for assistance in creating Fig. 5.1.

References Abellan, M.T., Adell, A., Honrubia, M.A., Mengod, G., Artigas, F., 2000. GABAB-RI receptors in serotonergic neurons: effects of baclofen on 5-HT output in rat brain. NeuroReport 11, 941e945. Abellan, M.T., Jolas, T., Aghajanian, G.K., Artigas, F., 2000. Dual control of dorsal raphe serotonergic neurons by GABA(B) receptors. Electrophysiological and microdialysis studies. Synapse 36, 21e34. Albert, P.R., Vahid-Ansari, F., Luckhart, C., 2014. Serotonin-prefrontal cortical circuitry in anxiety and depression phenotypes: pivotal role of pre- and post-synaptic 5-HT1A receptor expression. Frontiers in Behavioral Neuroscience 8, 199. Arranz, B., Cowburn, R., Eriksson, A., Vestling, M., Marcusson, J., 1992. Gamma-aminobutyric acid-B (GABAB) binding sites in postmortem suicide brains. Neuropsychobiology 26, 33e36.

References

75

Artigas, F., Romero, L., de Montigny, C., Blier, P., 1996. Acceleration of the effect of selected antidepressant drugs in major depression by 5-HT1A antagonists. Trends in Neurosciences 19, 378e383. Bangasser, D.A., Valentino, R.J., 2012. Sex differences in molecular and cellular substrates of stress. Cellular and Molecular Neurobiology 32, 709e723. Bannerman, D.M., Rawlins, J.N., McHugh, S.B., Deacon, R.M., Yee, B.K., Bast, T., Zhang, W.N., Pothuizen, H.H., Feldon, J., 2004. Regional dissociations within the hippocampus–memory and anxiety. Neuroscience and Biobehavioral Reviews 28, 273e283. Bergmann, O., Spalding, K.L., Frisen, J., 2015. Adult neurogenesis in humans. Cold Spring Harbor Perspectives in Biology 7, a018994. Berton, O., McClung, C.A., Dileone, R.J., Krishnan, V., Renthal, W., Russo, S.J., Graham, D., Tsankova, N.M., Bolanos, C.A., Rios, M., et al., 2006. Essential role of BDNF in the mesolimbic dopamine pathway in social defeat stress. Science 311, 864e868. Bettler, B., Kaupmann, K., Mosbacher, J., Gassmann, M., 2004. Molecular structure and physiological functions of GABA(B) receptors. Physiological Reviews 84, 835e867. Bischoff, S., Leonhard, S., Reymann, N., Schuler, V., Shigemoto, R., Kaupmann, K., Bettler, B., 1999. Spatial distribution of GABA(B)R1 receptor mRNA and binding sites in the rat brain. The Journal of Comparative Neurology 412, 1e16. Bjorkholm, C., Monteggia, L.M., 2016. BDNF - a key transducer of antidepressant effects. Neuropharmacology 102, 72e79. Blier, P., Ward, N.M., 2003. Is there a role for 5-HT1A agonists in the treatment of depression? Biological Psychiatry 53, 193e203. Blier, P., Bergeron, R., de Montigny, C., 1997. Selective activation of postsynaptic 5-HT1A receptors induces rapid antidepressant response. Neuropsychopharmacology: Official Publication of the American College of Neuropsychopharmacology 16, 333e338. Bravo, J.A., Forsythe, P., Chew, M.V., Escaravage, E., Savignac, H.M., Dinan, T.G., Bienenstock, J., Cryan, J.F., 2011. Ingestion of Lactobacillus strain regulates emotional behavior and central GABA receptor expression in a mouse via the vagus nerve. Proceedings of the National Academy of Sciences of the United States of America 108, 16050e16055. Brown, E.S., Rush, A.J., McEwen, B.S., 1999. Hippocampal remodeling and damage by corticosteroids: implications for mood disorders. Neuropsychopharmacology: Official Publication of the American College of Neuropsychopharmacology 21, 474e484. Challis, C., Boulden, J., Veerakumar, A., Espallergues, J., Vassoler, F.M., Pierce, R.C., Beck, S.G., Berton, O., 2013. Raphe GABAergic neurons mediate the acquisition of avoidance after social defeat. Journal of Neuroscience: The Official Journal of the Society for Neuroscience 33, 13978e13988, 13988a. Christian, K.M., Song, H., Ming, G.L., 2014. Functions and dysfunctions of adult hippocampal neurogenesis. Annual Review of Neuroscience 37, 243e262. Cornelisse, L.N., Van der Harst, J.E., Lodder, J.C., Baarendse, P.J., Timmerman, A.J., Mansvelder, H.D., Spruijt, B.M., Brussaard, A.B., 2007. Reduced 5-HT1A- and GABAB receptor function in dorsal raphe neurons upon chronic fluoxetine treatment of socially stressed rats. Journal of Neurophysiology 98, 196e204. Croarkin, P.E., Nakonezny, P.A., Husain, M.M., Port, J.D., Melton, T., Kennard, B.D., Emslie, G.J., Kozel, F.A., Daskalakis, Z.J., 2014. Evidence for pretreatment LICI deficits among depressed children and adolescents with nonresponse to fluoxetine. Brain Stimulation 7, 243e251. Cross, J.A., Horton, R.W., 1987. Are increases in GABAB receptors consistent findings following chronic antidepressant administration? European Journal of Pharmacology 141, 159e162. Cross, J.A., Horton, R.W., 1988. Effects of chronic oral administration of the antidepressants, desmethylimipramine and zimelidine on rat cortical GABAB binding sites: a comparison with 5-HT2 binding site changes. British Journal of Pharmacology 93, 331e336. Cross, J.A., Cheetham, S.C., Crompton, M.R., Katona, C.L., Horton, R.W., 1988. Brain GABAB binding sites in depressed suicide victims. Psychiatry Research 26, 119e129. Cryan, J.F., Kaupmann, K., 2005. Don’t worry ‘B’ happy!: a role for GABA(B) receptors in anxiety and depression. Trends in Pharmacological Sciences 26, 36e43.

76

5. GABAB receptors, depression, and stress resilience: a tale of two isoforms

Cryan, J.F., Leonard, B.E., 2000. 5-HT1A and beyond: the role of serotonin and its receptors in depression and the antidepressant response. Human Psychopharmacology 15, 113e135. Cryan, J.F., Slattery, D.A., 2010. GABAB receptors and depression. Current status. Advances in Pharmacology 58, 427e451. Cryan, J.F., Kelly, P.H., Chaperon, F., Gentsch, C., Mombereau, C., Lingenhoehl, K., Froestl, W., Bettler, B., Kaupmann, K., Spooren, W.P., 2004. Behavioral characterization of the novel GABAB receptor-positive modulator GS39783 (N,N’-dicyclopentyl-2-methylsulfanyl-5-nitro-pyrimidine-4,6-diamine): anxiolytic-like activity without side effects associated with baclofen or benzodiazepines. Journal of Pharmacology and Experimental Therapeutics 310, 952e963. De Vry, J., 1995. 5-HT1A receptor agonists: recent developments and controversial issues. Psychopharmacology 121, 1e26. Enna, S.J., Bowery, N.G., 2004. GABA(B) receptor alterations as indicators of physiological and pharmacological function. Biochemical Pharmacology 68, 1541e1548. Enna, S.J., Reisman, S.A., Stanford, J.A., 2006. CGP 56999A, a GABA(B) receptor antagonist, enhances expression of brain-derived neurotrophic factor and attenuates dopamine depletion in the rat corpus striatum following a 6-hydroxydopamine lesion of the nigrostriatal pathway. Neuroscience Letters 406, 102e106. Fanselow, M.S., Dong, H.W., 2010. Are the dorsal and ventral hippocampus functionally distinct structures? Neuron 65, 7e19. Felice, D., O’Leary OF, Cryan, J.F., 2016. Targeting the GABAB Receptor for the Treatment of Depression and Anxiety Disorders. GABAB Receptor. Springer International Publishing, pp. 219e250. Felice, D., O’Leary OF, Pizzo, R.C., Cryan, J.F., 2012. Blockade of the GABA(B) receptor increases neurogenesis in the ventral but not dorsal adult hippocampus: relevance to antidepressant action. Neuropharmacology 63, 1380e1388. Ferres-Coy, A., Santana, N., Castane, A., Cortes, R., Carmona, M.C., Toth, M., Montefeltro, A., Artigas, F., Bortolozzi, A., 2013. Acute 5-HT(1)A autoreceptor knockdown increases antidepressant responses and serotonin release in stressful conditions. Psychopharmacology 225, 61e74. Franklin, T.B., Saab, B.J., Mansuy, I.M., 2012. Neural mechanisms of stress resilience and vulnerability. Neuron 75, 747e761. Frankowska, M., Filip, M., Przegalinski, E., 2007. Effects of GABAB receptor ligands in animal tests of depression and anxiety. Pharmacological Reports 59, 645e655. Fritschy, J.M., Meskenaite, V., Weinmann, O., Honer, M., Benke, D., Mohler, H., 1999. GABAB-receptor splice variants GB1a and GB1b in rat brain: developmental regulation, cellular distribution and extrasynaptic localization. European Journal of Neuroscience 11, 761e768. Gassmann, M., Bettler, B., 2012. Regulation of neuronal GABA(B) receptor functions by subunit composition. Nature Reviews Neuroscience 13, 380e394. Ghose, S., Winter, M.K., McCarson, K.E., Tamminga, C.A., Enna, S.J., 2011. The GABAb receptor as a target for antidepressant drug action. British Journal of Pharmacology 162, 1e17. Giachino, C., Barz, M., Tchorz, J.S., Tome, M., Gassmann, M., Bischofberger, J., Bettler, B., Taylor, V., 2014. GABA suppresses neurogenesis in the adult hippocampus through GABAB receptors. Development 141, 83e90. Goel, N., Workman, J.L., Lee, T.T., Innala, L., Viau, V., 2014. Sex differences in the HPA axis. Comprehensive Physiology 4, 1121e1155. Gray, J.A., Green, A.R., 1987. Increased GABAB receptor function in mouse frontal cortex after repeated administration of antidepressant drugs or electroconvulsive shocks. British Journal of Pharmacology 92, 357e362. Hannan, S., Wilkins, M.E., Smart, T.G., 2012. Sushi domains confer distinct trafficking profiles on GABAB receptors. Proceedings of the National Academy of Sciences of the United States of America 109, 12171e12176. Heese, K., Otten, U., Mathivet, P., Raiteri, M., Marescaux, C., Bernasconi, R., 2000. GABA(B) receptor antagonists elevate both mRNA and protein levels of the neurotrophins nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) but not neurotrophin-3 (NT-3) in brain and spinal cord of rats. Neuropharmacology 39, 449e462. Henckens, M.J.A.G., Deussing, J.M., Chen, A., 2016. Region-specific roles of the corticotropin-releasing factorurocortin system in stress. Nature Reviews Neuroscience 17, 636e651.

References

77

Hensler, J.G., 2003. Regulation of 5-HT1A receptor function in brain following agonist or antidepressant administration. Life Sciences 72, 1665e1682. Hoyer, D., Hannon, J.P., Martin, G.R., 2002. Molecular, pharmacological and functional diversity of 5-HT receptors. Pharmacology, Biochemistry and Behavior 71, 533e554. Jacobson, L., Sapolsky, R., 1991. The role of the hippocampus in feedback regulation of the hypothalamic-pituitaryadrenocortical axis. Endocrine Reviews 12, 118e134. Jacobson, L.H., Hoyer, D., Fehlmann, D., Bettler, B., Kaupmann, K., Cryan, J.F., 2017. Blunted 5-HT1A receptormediated responses and antidepressant-like behavior in mice lacking the GABAB1a but not GABAB1b subunit isoforms. Psychopharmacology 234, 1511e1523. Kempermann, G., Song, H., Gage, F.H., 2015. Neurogenesis in the adult Hippocampus. Cold Spring Harbor Perspectives in Biology 7, a018812. Klempan, T.A., Sequeira, A., Canetti, L., Lalovic, A., Ernst, C., ffrench-Mullen, J., Turecki, G., 2009. Altered expression of genes involved in ATP biosynthesis and GABAergic neurotransmission in the ventral prefrontal cortex of suicides with and without major depression. Molecular Psychiatry 14, 175e189. Krishnan, V., Han, M.H., Graham, D.L., Berton, O., Renthal, W., Russo, S.J., Laplant, Q., Graham, A., Lutter, M., Lagace, D.C., et al., 2007. Molecular adaptations underlying susceptibility and resistance to social defeat in brain reward regions. Cell 131, 391e404. la Cour, C.M., Hanoun, N., Melfort, M., Hen, R., Lesch, K.P., Hamon, M., Lanfumey, L., 2004. GABA(B) receptors in 5-HT transporter- and 5-HT1A receptor-knock-out mice: further evidence of a transduction pathway shared with 5-HT1A receptors. Journal of Neurochemistry 89, 886e896. Lee, C., Mayfield, R.D., Harris, R.A., 2010. Intron 4 containing novel GABAB1 isoforms impair GABAB receptor function. PLoS One 5, e14044. Levinson, A.J., Fitzgerald, P.B., Favalli, G., Blumberger, D.M., Daigle, M., Daskalakis, Z.J., 2010. Evidence of cortical inhibitory deficits in major depressive disorder. Biological Psychiatry 67, 458e464. Levone, B.R., Cryan, J.F., O’Leary OF, 2015. Role of adult hippocampal neurogenesis in stress resilience. Neurobiology of Stress 1, 147e155. Li, X., Kaczanowska, K., Finn, M.G., Markou, A., Risbrough, V.B., 2015. The GABA(B) receptor positive modulator BHF177 attenuated anxiety, but not conditioned fear, in rats. Neuropharmacology 97, 357e364. Lloyd, K.G., Thuret, F., Pilc, A., 1985. Upregulation of gamma-aminobutyric acid (GABA) B binding sites in rat frontal cortex: a common action of repeated administration of different classes of antidepressants and electroshock. Journal of Pharmacology and Experimental Therapeutics 235, 191e199. Lucki, I., 1998. The spectrum of behaviors influenced by serotonin. Biological Psychiatry 44, 151e162. Malberg, J.E., Eisch, A.J., Nestler, E.J., Duman, R.S., 2000. Chronic antidepressant treatment increases neurogenesis in adult rat hippocampus. Journal of Neuroscience: The Official Journal of the Society for Neuroscience 20, 9104e9110. Malkesman, O., Scattoni, M.L., Paredes, D., Tragon, T., Pearson, B., Shaltiel, G., Chen, G., Crawley, J.N., Manji, H.K., 2010. The female urine sniffing test: a novel approach for assessing reward-seeking behavior in rodents. Biological Psychiatry 67, 864e871. Marchesi, C., Chiodera, P., De Ferri, A., De Risio, C., Dasso, L., Menozzi, P., Volpi, R., Coiro, V., 1991. Reduction of GH response to the GABA-B agonist baclofen in patients with major depression. Psychoneuroendocrinology 16, 475e479. Martin, P., Pichat, P., Massol, J., Soubrie, P., Lloyd, K.G., Puech, A.J., 1989. Decreased GABA B receptors in helpless rats e reversal by tricyclic antidepressants. Neuropsychobiology 22, 220e224. McManus, D.J., Greenshaw, A.J., 1991. Differential effects of antidepressants on GABAB and beta-adrenergic receptors in rat cerebral cortex. Biochemical Pharmacology 42, 1525e1528. Mombereau, C., Kaupmann, K., Froestl, W., Sansig, G., van der Putten, H., Cryan, J.F., 2004. Genetic and pharmacological evidence of a role for GABA(B) receptors in the modulation of anxiety- and antidepressant-like behavior. Neuropsychopharmacology: Official Publication of the American College of Neuropsychopharmacology 29, 1050e1062. Mombereau, C., Kaupmann, K., Gassmann, M., Bettler, B., van der Putten, H., Cryan, J.F., 2005. Altered anxiety and depression-related behaviour in mice lacking GABAB(2) receptor subunits. NeuroReport 16, 307e310. Nakagawa, Y., Sasaki, A., Takashima, T., 1999. The GABA(B) receptor antagonist CGP36742 improves learned helplessness in rats. European Journal of Pharmacology 381, 1e7.

78

5. GABAB receptors, depression, and stress resilience: a tale of two isoforms

Nowak, G., Partyka, A., Palucha, A., Szewczyk, B., Wieronska, J.M., Dybala, M., Metz, M., Librowski, T., Froestl, W., Papp, M., et al., 2006. Antidepressant-like activity of CGP 36742 and CGP 51176, selective GABAB receptor antagonists, in rodents. British Journal of Pharmacology 149, 581e590. O’Flynn, K., Dinan, T.G., 1993. Baclofen-induced growth hormone release in major depression: relationship to dexamethasone suppression test result. American Journal of Psychiatry 150, 1728e1730. O’Leary OF, Castren, E., 2010. Neurotrophic factors and antidepressant action: recent advances. In: Depression: From Psychopathology to Pharmacotherapy, pp. 199e223. O’Leary OF, Cryan, J.F., 2010. The behavioural genetics of serotonin: relevance to anxiety and depression. In: Handbook of the Behavioral Neurobiology of Serotonin. Academic Press, USA, pp. 749e791. O’Leary OF, Cryan, J.F., 2014. A ventral view on antidepressant action: roles for adult hippocampal neurogenesis along the dorsoventral axis. Trends in Pharmacological Sciences 35, 675e687. O’Leary OF, Felice, D., Galimberti, S., Savignac, H.M., Bravo, J.A., Crowley, T., El Yacoubi, M., Vaugeois, J.M., Gassmann, M., Bettler, B., et al., 2014. GABAB(1) receptor subunit isoforms differentially regulate stress resilience. Proceedings of the National Academy of Sciences of the United States of America 111, 15232e15237. O’Leary OF, O’Connor, R.M., Cryan, J.F., 2012. Lithium-induced effects on adult hippocampal neurogenesis are topographically segregated along the dorso-ventral axis of stressed mice. Neuropharmacology 62, 247e255. O’Leary OF, Zandy, S., Dinan, T.G., Cryan, J.F., 2013. Lithium augmentation of the effects of desipramine in a mouse model of treatment-resistant depression: a role for hippocampal cell proliferation. Neuroscience 228, 36e46. Pilc, A., Lloyd, K.G., 1984. Chronic antidepressants and GABA “B” receptors: a GABA hypothesis of antidepressant drug action. Life Sciences 35, 2149e2154. Pratt, G.D., Bowery, N.G., 1993. Repeated administration of desipramine and a gaba(B) receptor antagonist, cgp-36742, discretely up-regulates GABA(B) receptor-binding sites in rat frontal-cortex. British Journal of Pharmacology 110, 724e735. Reul, J.M., Collins, A., Saliba, R.S., Mifsud, K.R., Carter, S.D., Gutierrez-Mecinas, M., Qian, X., Linthorst, A.C., 2015. Glucocorticoids, epigenetic control and stress resilience. Neurobiology of Stress 1, 44e59. Russo, S.J., Nestler, E.J., 2013. The brain reward circuitry in mood disorders. Nature Reviews Neuroscience 14, 609e625. Saarelainen, T., Hendolin, P., Lucas, G., Koponen, E., Sairanen, M., MacDonald, E., Agerman, K., Haapasalo, A., Nawa, H., Aloyz, R., et al., 2003. Activation of the TrkB neurotrophin receptor is induced by antidepressant drugs and is required for antidepressant-induced behavioral effects. Journal of Neuroscience 23, 349e357. Sairanen, M., Lucas, G., Ernfors, P., Castren, M., Castren, E., 2005. Brain-derived neurotrophic factor and antidepressant drugs have different but coordinated effects on neuronal turnover, proliferation, and survival in the adult dentate gyrus. Journal of Neuroscience 25, 1089e1094. Sands, S.A., Reisman, S.A., Enna, S.J., 2003. Effects of stress and tranylcypromine on amphetamine-induced locomotor activity and GABA(B) receptor function in rat brain. Life Sciences 72, 1085e1092. Sands, S.A., Reisman, S.A., Enna, S.J., 2004. Effect of antidepressants on GABA(B) receptor function and subunit expression in rat hippocampus. Biochemical Pharmacology 68, 1489e1495. Slattery, D.A., Desrayaud, S., Cryan, J.F., 2005. GABAB receptor antagonist-mediated antidepressant-like behavior is serotonin-dependent. Journal of Pharmacology and Experimental Therapeutics 312, 290e296. Snyder, J.S., Soumier, A., Brewer, M., Pickel, J., Cameron, H.A., 2011. Adult hippocampal neurogenesis buffers stress responses and depressive behaviour. Nature 476, 458e461. Surget, A., Tanti, A., Leonardo, E.D., Laugeray, A., Rainer, Q., Touma, C., Palme, R., Griebel, G., Ibarguen-Vargas, Y., Hen, R., et al., 2011. Antidepressants recruit new neurons to improve stress response regulation. Molecular Psychiatry 16, 1177e1188. Suzdak, P.D., Gianutsos, G., 1986. Effect of chronic imipramine or baclofen on GABA-B binding and cyclic AMP production in cerebral cortex. European Journal of Pharmacology 131, 129e133. Sweeney, F.F., O’Leary OF, Cryan, J.F., 2013. GABAB receptor ligands do not modify conditioned fear responses in BALB/c mice. Behavioural Brain Research 256, 151e156. Szekely, A.M., Barbaccia, M.L., Costa, E., 1987. Effect of a protracted antidepressant treatment on signal transduction and [3H](-)-baclofen binding at GABAB receptors. Journal of Pharmacology and Experimental Therapeutics 243, 155e159.

References

79

Takahashi, A., Shimamoto, A., Boyson, C.O., DeBold, J.F., Miczek, K.A., 2010. GABA(B) receptor modulation of serotonin neurons in the dorsal raphe nucleus and escalation of aggression in mice. Journal of Neuroscience: The Official Journal of the Society for Neuroscience 30, 11771e11780. Tanti, A., Belzung, C., 2013. Neurogenesis along the septo-temporal axis of the hippocampus: are depression and the action of antidepressants region-specific? Neuroscience 252, 234e252. Tao, R., Ma, Z., Auerbach, S.B., 1996. Differential regulation of 5-hydroxytryptamine release by GABAA and GABAB receptors in midbrain raphe nuclei and forebrain of rats. British Journal of Pharmacology 119, 1375e1384. Varga, V., Sik, A., Freund, T.F., Kocsis, B., 2002. GABA(B) receptors in the median raphe nucleus: distribution and role in the serotonergic control of hippocampal activity. Neuroscience 109, 119e132. Vigot, R., Barbieri, S., Brauner-Osborne, H., Turecek, R., Shigemoto, R., Zhang, Y.P., Lujan, R., Jacobson, L.H., Biermann, B., Fritschy, J.M., et al., 2006. Differential compartmentalization and distinct functions of GABAB receptor variants. Neuron 50, 589e601. Vlachou, S., Markou, A., 2010. GABAB receptors in reward processes. Advances in Pharmacology 58, 315e371.

C H A P T E R

6

Sex differences in the programming of stress resilience 1

Kathleen E. Morrison1, 2, C. Neill Epperson3, Tracy L. Bale1, 2

Department of Pharmacology, University of Maryland School of Medicine, Baltimore, MD, United States; 2Center for Epigenetic Research in Child Health and Brain Development, University of Maryland School of Medicine, Baltimore, MD, United States; 3Department of Psychiatry, University of Colorado School of Medicine, Aurora, CO, United States

Introduction Sex is a critical factor in determining when an individual is vulnerable to stress, what type of stress is likely to produce long-term negative consequences, and in which behavioral domains the stress-induced dysfunction will manifest. Understanding how sex interacts with stress to impair psychological well-being is an important avenue of research. Exposure to chronic or extreme stressors is a major risk factor for neuropsychiatric disorders such as affective disorders, schizophrenia, autism spectrum disorder (ASD), and attention deficit hyperactivity disorder (ADHD), many of which are sex biased in their symptomology and prevalence (Tolin and Foa, 2006; Newschaffer et al., 2007; Erskine et al., 2013; Cover et al., 2014; Gore et al., 2014). Although stress researchers have long examined the negative consequences of stress, recent years have seen an increase in the study of resilience to stress. This is likely due to two factors: (1) epidemiological data demonstrate that whereas most humans will undergo some sort of traumatic or stressful life event, approximately an average of 10%e20% will develop long-term mental health disorders, although prevalence is dependent on age of insult, sex, and genetic factors, and (2) basic scientists are taking note of tremendous individual variation in how animals respond to stress and have begun to study these subgroups that would have potentially been overlooked in the past (Galea et al., 2005; Cohen et al., 2007; Thomas et al., 2010). Resilience is the capability to cope with adverse experiences such that an individual is not subject to the negative psychological and biological consequences that would otherwise lead

Stress Resilience https://doi.org/10.1016/B978-0-12-813983-7.00006-9

81

Copyright © 2020 Elsevier Inc. All rights reserved.

82

6. Sex differences in the programming of stress resilience

to biological dysfunction and increased disease risk (Russo et al., 2012; Cooper et al., 2015). Resilience may be demonstrated by resistance to the negative effects of stress or by recovery to a normal state of functioning more quickly than expected following a stressful event. It is important to distinguish between resistance to, and recovery from, stressful events, as these outcomes may involve distinct brain regions, neurochemical processes, and unique biomarkers. In this chapter, we use the term resilience to be inclusive of all levels of response. Throughout this chapter, it will become clear that resilience is not the same, psychologically or physiologically, as having not gone through the stressful life event (i.e., it is not the same as being unexposed or as being returned to “normal”). Indeed, resilience represents a third type of response that involves a discrete set of neural substrates and cellular mechanisms that enable individuals to avoid at least some of the negative consequences of extreme stress (Russo et al., 2012). The term resilience carries with it many assumptions mainly that the resilient group is in a more advantageous state than the vulnerable group. Although this may seem like the easiest explanation, it is important to remember that the utility of a biological response is context dependent. Therefore, an additional nuance of resilience is how it is contextualized. Take, for example, the outcome of an altered hypothalamic-pituitary-adrenal (HPA) axis response to stress, a common affective disorder endophenotype that is observed in preclinical animal models including that of prenatal or pubertal stress and in models of transgenerational transmission of stress dysregulation (Mueller and Bale, 2008; Rodgers et al., 2013; Morrison et al., 2017). Stress pathway dysregulation, as manifested in either increased or decreased reactivity, may reflect an organism’s inability to respond appropriately to a changing environment. Therefore, we might deem any individual who has a dysregulated HPA axis response following a stressful life event to be vulnerable, while those individuals who show no dysregulation are resilient. This interpretation makes sense considering the association between HPA axis dysfunction and disease state (Heim et al., 2008). However, an alternative interpretation may be that a change in the HPA axis response to stressors is adaptive. In the case of a reduced HPA axis response seen in the offspring of stressed males, a dampened HPA axis response to stress may reflect greater offspring fitness, particularly if the offspring’s environment is also stressful (Rodgers et al., 2013). In contrast, in the instance of pubertal adversity, both humans and mice show HPA axis dysregulation and alterations to maternal mood and behavior. Women are more likely to score at risk for postnatal depression, and mice show altered maternal behavior (Morrison et al., 2017). The findings in this instance indicate that the altered HPA axis is linked to the disease risk and reproductive status during which the stressor occurred, making relevant when the organism/individual is being tested. Therefore, it is important to contextualize why stressinduced changes might manifest as resilience or risk, and to fully characterize the other endpoints, such as depressive-like and anxiety-like behaviors, that pair with these physiological outcomes. It is also important to describe what is meant by sex differences in resilience. The mere presence of a sex difference in the behavioral or physiological response to a stimulus does not necessarily qualify one sex as “resilient” and the other as “vulnerable.” Instead, sex differences in resilience arise when some aspect of their biology places males and females on separate trajectories for the consequences of stress (Fig. 6.1). Take, for example, an arbitrary behavior where there is a known baseline sex difference in output of that

Introduction

83

FIGURE 6.1

Arbitrary behavioral responses are represented to demonstrate theoretical distinctions between a behavioral sex difference versus a sex difference in resilience to the effects of a stressful life event. Importantly, a sex difference in a behavioral response does not necessarily predict which sex will be resilient. (A) In this hypothetical example, stress-naïve females display double the amount of a behavior as stress-naïve males. When exposed to a stressful life event, there are several possibilities for resilience or risk outcomes. (B and C) On the one hand, it is possible that there will be no sex difference in how individuals respond to the stressful life event. (B) It is possible that neither males nor females will demonstrate any long-term behavioral change or that (C) both females and males will display the same magnitude of behavioral change in response to a stressful life event. (D and E) Alternatively, there may be a sex difference in how individuals respond. (D) Some stressful life events produce a behavioral effect only in males, and in this instance, females would be considered resilient. (E) The reverse can also be true, wherein females display a behavioral change and males are considered resilient.

behavior, such as rumination and self-blame in humans or risk-taking behavior in rodents (Fig. 6.1A) (Spindler et al., 2010; Johnson and Whisman, 2013; Jolles et al., 2015). Untested by the addition of a stressful life event, one is not able to determine whether males or females might be categorized as resilient. It is possible that there will be no sex difference in behavioral responding following a stressful life event, suggesting that males and females are equivalent in either their resilience (Fig. 6.1B) or risk (Fig. 6.1C). Alternatively, it is possible that males and females will differ in the consequences of a stressful life event, rendering either females (Fig. 6.1D) or males (Fig. 6.1E) as resilient. This example is presented in arbitrary units to illustrate many theoretical outcomes in stress-induced risk and resilience. Within this chapter, we will discuss specific examples of each of these possible outcomes. Furthermore, even though this is a nascent field, it is becoming very clear that the sex difference in programming resilience is highly nuanced. Labeling one sex with an outcome of “resilient” must be followed by qualifiers such as, to which type of stress, during which period of development, and evidenced in which behavioral or physiological outcome. In general, two areas under discussion in this chapter, sex differences and resilience, are relatively new as major focuses in the field of stress research. The negative consequences of stress on males have long been studied, and although there has been a small group of expert sex difference researchers, recent changes in expectations by major funding bodies and scientific journals have fueled research on sex as a biological variable for most researchers. We aim here to address the current state of the field and to propose new ideas and challenges to be met in the coming decades of research. It should be noted that we will be focusing on areas where sex differences are known and some understanding of the sex-specific programming has been pursued.

84

6. Sex differences in the programming of stress resilience

Sex x life span interaction in producing resilience Sex differences in the consequences of stress shift dramatically throughout the lifetime. In the prenatal period, females are likely to be resilient to the effects of a variety of stressors, including maternal trauma, maternal psychosocial stress, and maternal depression (Van Os and Selten, 1998; Khashan et al., 2008; Gerardin et al., 2011). Sex differences in resilience have been the most thoroughly studied during the prenatal window of development. One impetus for the wealth of studies that have been conducted during this time is the link between the risk for neuropsychiatric disorders and adverse exposures in utero and, during childhood, sensitive periods of early brain development. During the prenatal period, the brain is forming and is undergoing substantial and rapid development. Prenatal stress is a risk factor for neurodevelopmental disorders, including ASD, ADHD, and schizophrenia, which show a marked sex bias toward presentation in males, with the overall sex ratio at 4:1 for boys:girls in ASD and 3.2:1 for ADHD (Newschaffer et al., 2007; Erskine et al., 2013; Davis and Pfaff, 2014; Gore et al., 2014). Animal studies that utilize prenatal stress confirm that female offspring are resilient to negative outcomes such as stress axis dysregulation, cognitive deficits, behavioral reactivity, and metabolic issues, which are endophenotypes of neurodevelopmental disorder symptoms (Lemaire et al., 2000; Schneider et al., 2002; Kapoor and Matthews, 2005; Mueller and Bale, 2007, 2008). From birth to puberty, there are limited sex differences in the physiological stress response and presentation rate of affective disorders; however as discussed above, males are at greater risk for what is being referred to as neurodevelopmental disorders such as autism and ADHD (Kessler, 2003; Romeo and McEwen, 2006; Romeo, 2010). The directionality of sex differences in resilience to stress-induced affective dysfunction shifts in puberty and adolescence, when males are protected. Women are twice as likely as men to suffer from posttraumatic stress disorder (PTSD), and recent analyses suggest that this difference arises during puberty (Garza and Jovanovic, 2017). The same is true for depression and several anxiety disorders that emerge in adolescence, which is twice as high in females as in males (Wade et al., 2002; Hantsoo and Epperson, 2017). Importantly, there are sex differences in symptom severity in major depressive disorder (MDD) when adolescent onset is compared with adult onset. Individuals with adolescent-onset MDD are more likely to be women than those with adult-onset MDD, supporting the hypothesis that puberty and adolescent periods represent a time of increased risk in women. In all individuals with adolescent onset MDD, there was increased incidence of symptom severity including more suicide attempts compared with adult-onset MDD patients (Zisook et al., 2007). Despite these known relationships, puberty has been relatively understudied compared with early life in terms of stress reprogramming, especially in the area of sex differences. Animal models have produced somewhat conflicting results in the sex specificity of the effects of stress either during the onset of puberty or during the adolescent period, with some studies showing sex differences, some finding no difference in males and females, and some finding effects that are sex specific depending on the outcome (Toledo-Rodriguez and Sandi, 2011; Weathington et al., 2012; Harrell et al., 2013). These studies all utilize different stressors, applied for different lengths of time, and starting at slightly different ages. A significant issue for studying this period is the inconsistency across rodent models as to what constitutes puberty and adolescence. This is problematic for interpretation of findings, as well as for rigor and reproducibility. Development of the brain and

Sex x life span interaction in producing resilience

85

hormone axes during this period is rapid and dynamic, which may also account for disparities in animal research. In humans, adolescence is more protracted and associated with a peak and subsequent decline in cortical gray matter and a continual and sexually dimorphic increase in cortical white matter volume in both the frontal and parietal lobes by early adulthood (Pfefferbaum et al., 1994; Giedd et al., 1999; Perrin et al., 2008; Gennatas et al., 2017). Furthermore, the development of important limbic brain areas, including the prefrontal cortex, hippocampus, and amygdala, which are known to be disrupted in neuropsychiatric disorders, has been demonstrated across adolescence in animal models (Lee et al., 2003; Isgor et al., 2004; Matsuoka et al., 2010; Scherf et al., 2013). When the stressful life event occurs in adulthood, the sex difference in resilience is less pronounced, perhaps because adults are generally more resilient to the same stressors that will produce long-term consequences in younger individuals. Furthermore, although adults are still susceptible to acute consequences of stress, lasting 24e48 h following stress, they are less likely to be subject to the long-term reprogramming observed in younger and aged populations. In animal studies, stressors that produce dysfunction when experienced earlier in life are less likely to have any major or lasting effects on behavior in adults (Belda et al., 2004; Lupien et al., 2009). This may be because nervous system development plateaus in adulthood. Although the brain is still sensitive to input, and will therefore respond to stress as it is happening, it is more likely to return to baseline following the end of stress (Lupien et al., 2009). Even stressors that can produce more long-lasting effects, such as ethological stressors like social defeat stress, will not produce a permanent change in behavior among adults. In hamsters, it has been demonstrated that the effect of social defeat stress can last up to 33 days, and although this is relatively long-lasting, it is not a lifetime effect (Huhman et al., 2003). Alternatively, there are models such as learned helplessness, where exposure to uncontrollable or inescapable shock in adults produces a robust change to a suite of behaviors. In this instance, the effects are found to dissipate by 72 h following stressor exposure (Hammack et al., 2012). There are exceptions to these findings, most notably in the case of PTSD. PTSD is triggered by exposure to a severe traumatic event, and a single event can produce a lifetime of negative consequences. However, PTSD only presents in a subset of individuals that are exposed to trauma, and individual differences in biological responses such as the HPA axis are being examined as an underlying component of disease risk (Rodgers and Bale, 2015). Adulthood can then be a prime example of when the “two-hit” hypothesis of stress susceptibility or the concept of allostatic load, wherein an individual needs two risk factors, such as genetic risk, prior stressful experience, vulnerable period of brain development, or some physiological risk factor (see pregnancy below), to manifest as disease (Nederhof and Schmidt, 2012; Kuhn et al., 2016). Indeed, studies have shown that some amount of stress, whether in a controllable situation or at a subthreshold level for leading to disease, can produce better coping and resilience in the future (Karatsoreos et al., 2013). One example of the vulnerable window for risk and resilience of adults is during pregnancy and the postpartum period, when risk for affective disturbance is revealed in up to 20% of women (Dorn and Chrousos, 1997; Babb et al., 2015). Peripartum depression and anxiety are associated with significant adverse and long-term effects for both mother and baby (Gavin et al., 2005; Borri et al., 2008; Vesga-López et al., 2008). In consideration of the “two-hit” hypothesis, pregnancy can be thought of as an additional hit of stress that can interact with factors such as early-life stress or stress during pregnancy and postpartum to

86

6. Sex differences in the programming of stress resilience

produce vulnerability to long-term negative outcomes. Although there are limited animal studies that focus on pregnancy and the maternal outcome, as opposed to how stress during pregnancy impacts the offspring, their results do support clinical findings. During pregnancy, females experience another period of nervous system vulnerability, where stressful experiences can produce long-term negative outcomes (Brummelte et al., 2006). The mechanisms of brain vulnerability during pregnancy are understudied, although it has been hypothesized that increased levels of hormones and alterations in the immune system are key factors (Sherer et al., 2017). Chronic stress or glucocorticoid exposure during pregnancy and postpartum produces disruptions in maternal behavior and lasting alterations in hippocampal plasticity of the dam (Brummelte et al., 2006; Nephew and Bridges, 2011). There are also data in rodents showing that a negative experience during one pregnancy can influence the behavior of females during subsequence pregnancies. For example, elevated stress hormones during one pregnancy can alter postpartum behavior during a second pregnancy, even in the absence of any kind of negative stimulus in the index pregnancy or postnatal period (Wong et al., 2011). These preclinical data suggest that pregnancy and postpartum are windows of vulnerability for stressful life experiences to precipitate lasting dysfunction in brain and behavior. Therefore, entering into pregnancy and postpartum can transiently decrease the typical resilience observed in adult animals, rendering females vulnerable to stressful life experiences. In aged populations, we see again that males are more likely to be resilient to the effects of stress on cognitive decline and affective disturbance. However, risk for disturbance increases in women. Women are two to three times more likely to experience first onset depression during perimenopause, and late-onset schizophrenia is two times higher in women than in men (Nemeroff, 2007; Freeman et al., 2014). In the aging brain, reproductive senescence, particularly the perimenopausal period, leads to another vulnerable period for sensitivity to the effects of a stressful life event, especially in women. Biological functions such as the HPA axis are altered in a sex-specific way in aging. Older women show increased cortisol in response to a variety of stressors, including a cognitive challenge, pharmacological challenge, or psychological challenge, compared with age-matched men as well as younger men and women (Seeman et al., 2001; Kudielka et al., 2004; Kudielka and Kirschbaum, 2005; Otte et al., 2005). As the HPA axis is disrupted in many affective disorders, this shift in responsiveness might represent an increased vulnerability in perimenopausal women. There have been similar findings in animal studies of the HPA axis during aging, where aged female rats have higher basal corticosterone compared with age-matched males (Bowman et al., 2006). The decrease in estrogen during aging has been associated with issues of cognitive function, potentially through alterations to synaptic plasticity. For example, neural plasticity, as evidenced by hippocampal and hypothalamic synaptogenesis, is sensitive to estrogen (Woolley, 2007). Animal studies have shown that middle-aged female mice are more susceptible to stress effects on neurogenesis in the hippocampus than males (Tzeng et al., 2016). How is it that the sex of an individual produces such a dynamic change in resilience throughout the life span? The ability to conduct mechanistic studies that examine the drivers of sex-specific nervous system development have pointed to the role of gonadal hormones and sex chromosome complement as key factors in nervous system development and resilience to stress.

Sex hormone x life span interaction in producing resilience

87

Sex hormone x life span interaction in producing resilience Sex differences in nervous system development seem to arise from several sources. The role of gonadal hormones in producing sex differences in brain development and behavior is well established (MacLusky and Naftolin, 1981; McCarthy et al., 2012). The classic view of this is known as the “activational and organizational” hypothesis, whereby exposure to different gonadal hormones, androgens in males and estrogens in females, programs permanent sex-specific development and behavior. During the prenatal and perinatal window, males experience a testicular testosterone surge that is responsible for organizing the brain to respond to future activational effects of hormones, such as that occurs at puberty and throughout adulthood, to produce male-specific brain development and behaviors (Phoenix et al., 1959). For many decades following the establishment of the “organizational and activational” hypothesis, it was accepted dogma that the hormone surge that occurs during puberty merely “turns on” the sex differences that were programmed perinatally. However, more recent studies have shed light on the discovery that the onset of hormones in puberty also organizes brain and behavior (Romeo, 2003; Schulz et al., 2009). When gonadal hormone exposure in puberty is delayed, there are irreversible effects on brain maturation. Castrated males show deficits in masculine behavior that cannot be recovered by later testosterone treatment or sexual experience. This extends to other behaviors such as flank-marking, which is dependent on testosterone exposure. Castration prior to puberty results in disruption in flank-marking and the neural circuits that underlie the behavior. If males were castrated prior to the start of puberty, adult exposure to testosterone could not recover the deficits in flankmarking behavior. Similar studies have been done in females, demonstrating that ovariectomy prior to puberty disrupts lordosis behavior in adults (Schulz et al., 2004, 2006; Schulz and Sisk, 2006). Therefore, aging represents another period where there is a sex difference in resilience to stress. Males may be categorized as resilient, although this is only relative to females who are really demonstrating increased vulnerability. Gonadal testosterone contributes to prenatal and perinatal sex-specific brain development. Animal studies have shown that aromatization of gonadal testosterone to estradiol in the brain drives masculinization, which affects cell differentiation and brain connectivity (McCarthy and Arnold, 2011; McCarthy and Nugent, 2013). This estradiol is critical in directing cell death and cell birth in the developing nervous system, especially in sexually dimorphic brain regions (Morgan and Bale, 2012; McCarthy and Nugent, 2013). In females, other processes are important for feminizing sex-specific regions of the brain, including DNA methylation to repress masculinization (Nugent et al., 2015). Although the role for gonad-derived sex hormones in brain development is clear, recent work has uncovered critical new sources of these hormones during gestation. Even prior to the presence of testicular testosterone during the perinatal period, the placenta provides the developing fetus with steroid hormones. The placenta is a particularly interesting candidate for determining sex differences in resilience. The placenta is the barrier between the maternal and fetal compartments, determining what signals pass through to the fetal compartment (Nugent and Bale, 2015). The placenta has robust steroidogenic activity, and the male placenta produces high levels of testosterone during gestation. This testosterone exposure has been implicated in the vulnerability of males to prenatal stress. Clinical studies

88

6. Sex differences in the programming of stress resilience

show a correlation between fetal testosterone and alterations in steroidogenic activity in the placenta that are characteristic of neurodevelopmental disorders (Ruta et al., 2011; Lombardo et al., 2012; Gore et al., 2014; Baron-Cohen et al., 2015). Therefore, female resilience to prenatal stress may be in part derived from the fact that female placentas are not sensitive to disruptions in steroidogenic activity. The surge of sex hormones in puberty also influences physiological processes that determine how males and females respond to stress. The responsiveness of the HPA axis in prepubescent animals is different from that of the fully matured adult response. In response to a variety of stimuli, male and female prepubescent rodents have an HPA axis characterized by a protracted hormonal response compared with neonatal and adult animals, and an insensitivity to factors, such as gonadal hormones, that normally modulate the adult response (Romeo et al., 2004a,b, 2013). Following the rise in hormones that is triggered by the reemergence of gonadotropin-releasing hormone, the HPA axis, as well as other important processes including sex-specific cell proliferation in several brain regions, begins to mature into the adult, sex-dependent phenotype. Although it is known that gonadal hormones are critical in organizing the pubertal brain, more work needs to be done to directly address the role that these hormones play in stress resilience. Still another relative mystery is the mechanism by which extraordinarily high hormone levels during pregnancy could produce a stress vulnerability that is not normally seen in adult females. Neuroactive steroids such as estradiol, progesterone, and allopregnanolone are dramatically increased in the periphery and in the brain, where they can modulate the function of multiple neurotransmitter systems. Within 3 days of childbirth, these same neuroactive steroids drop to postmenopausal levels. These dynamic changes in steroid levels require the female brain to be flexible and resilient. The complex questions of the mechanisms of risk for affective dysfunction during pregnancy represent an important area for future research, as maternal mental health is being recognized as an increasingly significant concern in the lifetime of women. The sex difference in resilience in aging, when females are more vulnerable to stress than males, is explained in part by the onset of reproductive senescence in females when gonadal steroids shift dramatically and somewhat erratically for years and then decline to hypogonadal levels during the postmenopause. In contrast, males experience less dramatic or no change in gonadal hormone levels until much later in life. These hormonal fluctuations and resultant hypogonadism are likely contributors to the reduced resilience in aging females, as estradiol and progesterone are key molecules in maintaining brain structure and function, including executive function, learning and memory, and stress regulation (Shanmugan and Epperson, 2014). As with puberty and pregnancy, the exact nature of the mechanism linking reduced gonadal hormone levels to vulnerability in aged females has yet to be examined.

Sex chromosome x life span interaction in producing resilience As gonadal hormones are critical to determining sex differences in the brain, it is important to understand what drives sex differences in gonadal hormone secretion. Sex chromosome complement, XX in females and XY in males, is the determining genetic factor for sex of an individual and the underlying contributor in many sex-specific biological processes,

Conclusion

89

including gonadal hormone secretion. As described above, a long-held view of brain development is that exposure to gonadal hormones is the main driver of sexual differentiation. In recent years, it has also been shown that sex chromosome complement produces organizational sex differences in the brain that are independent of gonadal hormone levels. This work has been achieved with the use of the “four-core” genotype mice, a line of mice where the testes determining factor gene, Sry, has been transposed onto an autosome, producing gonadal females (XX or XY , with ovaries) and males (XY, XY Sry, or XXSry, with testes) (De Vries et al., 2002). Studies utilizing these mice have demonstrated a role for sex chromosomes that is dissociable from the action of gonadal hormones in brain maturation. For example, sex differences in the vasopressin system are more masculinized XY Sry males than XXSry males, indicating an effect of sex chromosome complement beyond the fact that both of these types of males have testes (De Vries et al., 2002). Although the specific role for sex chromosomes in stress resilience has not been examined, the work with the four-core model suggests that sex chromosome complement is critical in guiding sexspecific development of brain and behavior. Developmentally, sex chromosomes play a critical role very early in gestation, including an important role in the function of the placenta. The placenta is a tissue of fetal origin, and therefore the fetal aspect of the placenta carries the sex chromosome complement of the fetus. Sex chromosome complement is a critical determinant in the response of the placenta to maternal insults, where the female placenta seems to provide a protective effect (Nugent and Bale, 2015). Genes on the sex chromosomes are expressed early in placental differentiation in rodents and humans, providing sex-specific transplacental signals. Importantly, genes linked to sex chromosomes play a role in female resilience to maternal stress (Bale, 2016). In an established mouse model of early prenatal stress (EPS), male, but not female, offspring demonstrate increased stress sensitivity as adults (Mueller and Bale, 2008). Through genome-wide screening for EPS-induced changes in the placenta, the X-linked gene O-linked N-acetylglucosamine transferase (OGT) was identified as a top candidate for regulating the cellular response to changes in the maternal milieu. Subsequent mechanistic studies confirmed that lower levels of OGT in males promote an increased risk for stress-induced changes in neurodevelopmental programming and metabolic regulation (Howerton et al., 2013; Howerton and Bale, 2014). Therefore, the increased level of OGT within the female placenta appears to provide a resiliency factor to the effects of maternal insults. This provides one very early developmental mechanism whereby sex chromosomes are a critical factor in prenatal female resilience.

Conclusion As stress is an inevitable and seemingly increasing part of daily life, the opportunity to study what programs resilience, as well as risk, is an important direction of scientific research. By studying resilient populations, we can learn (1) what types of experiences counteract the effects of stress and (2) the genetic, biochemical, and molecular signature of resilience (Lyons et al., 2010; Drury et al., 2016). The nascent field of studying sex differences in resilience has already provided evidence that it is important to understand how males and females respond differently to stress, as has been discussed in this chapter.

90

6. Sex differences in the programming of stress resilience

Acknowledgments This work was supported by the National Institutes of Health grants HD091376 (KEM), MH099910 (TLB and CNE), ES028202 (TLB), MH104184 (TLB), and MH108286 (TLB).

References Babb, J.A., Deligiannidis, K.M., Murgatroyd, C.A., Nephew, B.C., 2015. Peripartum depression and anxiety as an integrative cross domain target for psychiatric preventative measures. Behavioural Brain Research 276, 32e44. Bale, T.L., 2016. The placenta and neurodevelopment: sex differences in prenatal vulnerability. Dialogues in Clinical Neuroscience 18, 459e464. Baron-Cohen, S., Auyeung, B., Nørgaard-Pedersen, B., Hougaard, D.M., Abdallah, M.W., Melgaard, L., Cohen, A.S., Chakrabarti, B., Ruta, L., Lombardo, M.V., 2015. Elevated fetal steroidogenic activity in autism. Molecular Psychiatry 20, 369e376. Belda, X., Márquez, C., Armario, A., 2004. Long-term effects of a single exposure to stress in adult rats on behavior and hypothalamic-pituitary-adrenal responsiveness: comparison of two outbred rat strains. Behavioural Brain Research 154, 399e408. Borri, C., Mauri, M., Oppo, A., Banti, S., Rambelli, C., Ramacciotti, D., Montagnani, M.S., Camilleri, V., Cortopassi, S., Bettini, A., Ricciardulli, S., Rucci, P., Montaresi, S., Cassano, G.B., 2008. Axis I psychopathology and functional impairment at the third month of pregnancy: results from the Perinatal Depression-Research and Screening Unit (PND-ReScU) study. Journal of Clinical Psychiatry 69, 1617e1624. Bowman, R.E., Maclusky, N.J., Diaz, S.E., Zrull, M.C., Luine, V.N., 2006. Aged rats: sex differences and responses to chronic stress. Brain Research 1126, 156e166. Brummelte, S., Pawluski, J.L., Galea, L.A., 2006. High post-partum levels of corticosterone given to dams influence postnatal hippocampal cell proliferation and behavior of offspring: a model of post-partum stress and possible depression. Hormones and Behavior 50, 370e382. Cohen, S., Janicki-Deverts, D., Miller, G.E., 2007. Psychological stress and disease. Journal of the American Medical Association 298, 1685e1687. Cooper, M.A., Clinard, C.T., Morrison, K.E., 2015. Neurobiological mechanisms supporting experience-dependent resistance to social stress. Neuroscience 291, 1e14. Cover, K.K., Maeng, L.Y., Lebrón-Milad, K., Milad, M.R., 2014. Mechanisms of estradiol in fear circuitry: implications for sex differences in psychopathology. Translational Psychiatry 4, e422. Davis, E.P., Pfaff, D., 2014. Sexually dimorphic responses to early adversity: implications for affective problems and autism spectrum disorder. Psychoneuroendocrinology 49, 11e25. De Vries, G.J., Rissman, E.F., Simerly, R.B., Yang, L.-Y.Y., Scordalakes, E.M., Auger, C.J., Swain, A., Lovell-Badge, R., Burgoyne, P.S., Arnold, A.P., 2002. A model system for study of sex chromosome effects on sexually dimorphic neural and behavioral traits. Journal of Neuroscience 22, 9005e9014. Dorn, L.D., Chrousos, G.P., 1997. The neurobiology of stress: understanding regulation of affect during female biological transitions. Seminars in Reproductive Endocrinology 15, 19e35. Drury, S.S., Sánchez, M.M., Gonzalez, A., 2016. When mothering goes awry: challenges and opportunities for utilizing evidence across rodent, nonhuman primate and human studies to better define the biological consequences of negative early caregiving. Hormones and Behavior 77, 182e192. Erskine, H.E., Ferrari, A.J., Nelson, P., Polanczyk, G.V., Flaxman, A.D., Vos, T., Whiteford, H.A., Scott, J.G., 2013. Epidemiological modelling of attention-deficit/hyperactivity disorder and conduct disorder for the Global Burden of Disease Study 2010. Journal of Child Psychology and Psychiatry 54, 1263e1274. Freeman, E.W., Sammel, M.D., Boorman, D.W., Zhang, R., 2014. Longitudinal pattern of depressive symptoms around natural menopause. Journal of the American Medical Association. Available at: http://jamanetwork. com/journals/jamapsychiatry/fullarticle/1772342. Galea, S., Nandi, A., Vlahov, D., 2005. The epidemiology of post-traumatic stress disorder after disasters. Epidemiologic Reviews 27, 78e91. Garza, K., Jovanovic, T., 2017. Impact of gender on child and adolescent PTSD. Current Psychiatry Reports 19, 87. Gavin, N.I., Gaynes, B.N., Lohr, K.N., Meltzer-Brody, S., Gartlehner, G., Swinson, T., 2005. Perinatal depression: a systematic review of prevalence and incidence. Obstetrics and Gynecology 106, 1071e1083.

References

91

Gennatas, E.D., Avants, B.B., Wolf, D.H., Satterthwaite, T.D., Ruparel, K., Ciric, R., Hakonarson, H., Gur, R.E., Gur, R.C., 2017. Age-related effects and sex differences in gray matter density, volume, mass, and cortical thickness from childhood to young adulthood. Journal of Neuroscience 37, 5065e5073. Gerardin, P., Wendland, J., Bodeau, N., Galin, A., Bialobos, S., Tordjman, S., Mazet, P., Darbois, Y., Nizard, J., Dommergues, M., Cohen, D., 2011. Depression during pregnancy: is the developmental impact earlier in boys? A prospective case-control study. Journal of Clinical Psychiatry 72, 378e387. Giedd, J.N., Blumenthal, J., Jeffries, N.O., Castellanos, F.X., Liu, H., Zijdenbos, A., Paus, T., Evans, A.C., Rapoport, J.L., 1999. Brain development during childhood and adolescence: a longitudinal MRI study. Nature Neuroscience 2, 861e863. Gore, A.C., Martien, K.M., Gagnidze, K., Pfaff, D., 2014. Implications of prenatal steroid perturbations for neurodevelopment, behavior, and autism. Endocrine Reviews 35, 961e991. Hammack, S.E., Cooper, M.A., Lezak, K.R., 2012. Overlapping neurobiology of learned helplessness and conditioned defeat: implications for PTSD and mood disorders. Neuropharmacology 62, 565e575. Hantsoo, L., Epperson, C.N., 2017. Anxiety disorders among women: a female lifespan approach. Focus (American Psychiatric Publishing) 15, 162e172. Harrell, C.S., Hardy, E., Boss-Williams, K., Weiss, J.M., Neigh, G.N., 2013. Sex and lineage interact to predict behavioral effects of chronic adolescent stress in rats. Behavioural Brain Research 248, 57e61. Heim, C., Newport, D.J., Mletzko, T., Miller, A.H., Nemeroff, C.B., 2008. The link between childhood trauma and depression: insights from HPA axis studies in humans. Psychoneuroendocrinology 33, 693e710. Howerton, C.L., Bale, T.L., 2014. Targeted placental deletion of OGT recapitulates the prenatal stress phenotype including hypothalamic mitochondrial dysfunction. Proceedings of the National Academy of Sciences of the United States of America 111, 9639e9644. Howerton, C.L., Morgan, C.P., Fischer, D.B., Bale, T.L., 2013. O-GlcNAc transferase (OGT) as a placental biomarker of maternal stress and reprogramming of CNS gene transcription in development. Proceedings of the National Academy of Sciences of the United States of America 110, 5169e5174. Huhman, K.L., Solomon, M.B., Janicki, M., Harmon, A.C., Lin, S.M., Israel, J.E., Jasnow, A.M., 2003. Conditioned defeat in male and female Syrian hamsters. Hormones and Behavior 44, 293e299. Isgor, C., Kabbaj, M., Akil, H., Watson, S.J., 2004. Delayed effects of chronic variable stress during peripubertaljuvenile period on hippocampal morphology and on cognitive and stress axis functions in rats. Hippocampus 14, 636e648. Johnson, D.P., Whisman, M.A., 2013. Gender differences in rumination: a meta-analysis. Personality and Individual Differences 55, 367e374. Jolles, J.W., Boogert, N.J., van den Bos, R., 2015. Sex differences in risk-taking and associative learning in rats. Royal Society Open Science 2, 150485. Kapoor, A., Matthews, S.G., 2005. Short periods of prenatal stress affect growth, behaviour and hypothalamopituitary-adrenal axis activity in male guinea pig offspring. Journal of Physiology (London) 566, 967e977. Karatsoreos, I.N., Karatoreos, I.N., McEwen, B.S., 2013. Annual research review: the neurobiology and physiology of resilience and adaptation across the life course. Journal of Child Psychology and Psychiatry 54, 337e347. Kessler, R.C., 2003. Epidemiology of women and depression. Journal of Affective Disorders 74, 5e13. Khashan, A.S., Abel, K.M., McNamee, R., Pedersen, M.G., Webb, R.T., Baker, P.N., Kenny, L.C., Mortensen, P.B., 2008. Higher risk of offspring schizophrenia following antenatal maternal exposure to severe adverse life events. Archives of General Psychiatry 65, 146e152. Kudielka, B.M., Kirschbaum, C., 2005. Sex differences in HPA axis responses to stress: a review. Biological Psychology 69, 113e132. Kudielka, B.M., Schommer, N.C., Hellhammer, D.H., Kirschbaum, C., 2004. Acute HPA axis responses, heart rate, and mood changes to psychosocial stress (TSST) in humans at different times of day. Psychoneuroendocrinology 29, 983e992. Kuhn, M., Scharfenort, R., Schümann, D., Schiele, M.A., Münsterkötter, A.L., Deckert, J., Domschke, K., Haaker, J., Kalisch, R., Pauli, P., Reif, A., Romanos, M., Zwanzger, P., Lonsdorf, T.B., 2016. Mismatch or allostatic load? Timing of life adversity differentially shapes gray matter volume and anxious temperament. Social Cognitive and Affective Neuroscience 11, 537e547. Lee, P.R., Brady, D., Koenig, J.I., 2003. Corticosterone alters N-methyl-D-aspartate receptor subunit mRNA expression before puberty. Brain Research. Molecular Brain Research 115, 55e62.

92

6. Sex differences in the programming of stress resilience

Lemaire, V., Koehl, M., Le Moal, M., Abrous, D.N., 2000. Prenatal stress produces learning deficits associated with an inhibition of neurogenesis in the hippocampus. Proceedings of the National Academy of Sciences of the United States of America 97, 11032e11037. Lombardo, M.V., Ashwin, E., Auyeung, B., Chakrabarti, B., Taylor, K., Hackett, G., Bullmore, E.T., Baron-Cohen, S., 2012. Fetal testosterone influences sexually dimorphic gray matter in the human brain. Journal of Neuroscience 32, 674e680. Lupien, S.J., McEwen, B.S., Gunnar, M.R., Heim, C., 2009. Effects of stress throughout the lifespan on the brain, behaviour and cognition. Nature Reviews Neuroscience 10, 434e445. Lyons, D.M., Parker, K.J., Schatzberg, A.F., 2010. Animal models of early life stress: implications for understanding resilience. Developmental Psychobiology 52, 616e624. MacLusky, N.J., Naftolin, F., 1981. Sexual differentiation of the central nervous system. Science 211, 1294e1302. Matsuoka, T., Sumiyoshi, T., Tsunoda, M., Takasaki, I., Tabuchi, Y., Uehara, T., Itoh, H., Suzuki, M., Kurachi, M., 2010. Change in the expression of myelination/oligodendrocyte-related genes during puberty in the rat brain. Journal of Neural Transmission (Vienna, Austria: 1996) 117, 1265e1268. McCarthy, M.M., Arnold, A.P., Ball, G.F., Blaustein, J.D., De Vries, G.J., 2012. Sex differences in the brain: the not so inconvenient truth. The Journal of Neuroscience. Available at: http://www.jneurosci.org/content/32/7/2241. short. McCarthy, M., Arnold, A., 2011. Reframing sexual differentiation of the brain. Nature Neuroscience 14, 677e683. McCarthy, M.M., Nugent, B.M., 2013. Epigenetic contributions to hormonally-mediated sexual differentiation of the brain. Journal of Neuroendocrinology 25, 1133e1140. Morgan, C.P., Bale, T.L., 2012. Sex differences in microRNA regulation of gene expression: no smoke, just miRs. Biology of Sex Differences 3, 22. Morrison, K.E., Epperson, C.N., Sammel, M.D., Ewing, G., Podcasy, J.S., Hantsoo, L., Kim, D.R., Bale, T.L., 2017. Preadolescent adversity programs a disrupted maternal stress reactivity in humans and mice. Biological Psychiatry 81, 693e701. Mueller, B.R., Bale, T.L., 2007. Early prenatal stress impact on coping strategies and learning performance is sex dependent. Physiology and Behavior 91, 55e65. Mueller, B.R., Bale, T.L., 2008. Sex-specific programming of offspring emotionality after stress early in pregnancy. Journal of Neuroscience 28, 9055e9065. Nederhof, E., Schmidt, M.V., 2012. Mismatch or cumulative stress: toward an integrated hypothesis of programming effects. Physiology and Behavior 106, 691e700. Nemeroff, C.B., 2007. Stress, menopause and vulnerability for psychiatric illness. Expert Review of Neurotherapeutics 7, S11eS13. Nephew, B.C., Bridges, R.S., 2011. Effects of chronic social stress during lactation on maternal behavior and growth in rats. Stress 14, 677e684. Newschaffer, C.J., Croen, L.A., Daniels, J., Giarelli, E., Grether, J.K., Levy, S.E., Mandell, D.S., Miller, L.A., PintoMartin, J., Reaven, J., Reynolds, A.M., Rice, C.E., Schendel, D., Windham, G.C., 2007. The epidemiology of autism spectrum disorders. Annual Review of Public Health 28, 235e258. Nugent, B.M., Bale, T.L., 2015. The omniscient placenta: metabolic and epigenetic regulation of fetal programming. Frontiers in Neuroendocrinology 39, 28e37. Nugent, B.M., Wright, C.L., Shetty, A.C., Hodes, G.E., Lenz, K.M., Mahurkar, A., Russo, S.J., Devine, S.E., McCarthy, M.M., 2015. Brain feminization requires active repression of masculinization via DNA methylation. Nature Neuroscience 18, 690e697. Otte, C., Hart, S., Neylan, T.C., Marmar, C.R., Yaffe, K., Mohr, D.C., 2005. A meta-analysis of cortisol response to challenge in human aging: importance of gender. Psychoneuroendocrinology 30, 80e91. Perrin, J.S., Hervé, P.-Y.Y., Leonard, G., Perron, M., Pike, G.B., Pitiot, A., Richer, L., Veillette, S., Pausova, Z., Paus, T., 2008. Growth of white matter in the adolescent brain: role of testosterone and androgen receptor. Journal of Neuroscience 28, 9519e9524. Pfefferbaum, A., Mathalon, D.H., Sullivan, E.V., Rawles, J.M., Zipursky, R.B., Lim, K.O., 1994. A quantitative magnetic resonance imaging study of changes in brain morphology from infancy to late adulthood. Archives of Neurology 51, 874e887. Phoenix, C.H., Goy, R.W., Gerall, A.A., Young, W.C., 1959. Organizing action of prenatally administered testosterone propionate on the tissues mediating mating behavior in the female guinea pig. Endocrinology 65, 369e382.

References

93

Rodgers, A.B., Bale, T.L., 2015. Germ cell origins of posttraumatic stress disorder risk: the transgenerational impact of parental stress experience. Biological Psychiatry 78, 307e314. Rodgers, A.B., Morgan, C.P., Bronson, S.L., Revello, S., Bale, T.L., 2013. Paternal stress exposure alters sperm microRNA content and reprograms offspring HPA stress axis regulation. Journal of Neuroscience 33, 9003e9012. Romeo, R.D., 2003. Puberty: a period of both organizational and activational effects of steroid hormones on neurobehavioural development. Journal of Neuroendocrinology 15, 1185e1192. Romeo, R.D., 2010. Pubertal maturation and programming of hypothalamic-pituitary-adrenal reactivity. Frontiers in Neuroendocrinology 31, 232e240. Romeo, R.D., Kaplowitz, E.T., Ho, A., Franco, D., 2013. The influence of puberty on stress reactivity and forebrain glucocorticoid receptor levels in inbred and outbred strains of male and female mice. Psychoneuroendocrinology 38, 592e596. Romeo, R.D., Lee, S.J., Chhua, N., McPherson, C.R., McEwen, B.S., 2004a. Testosterone cannot activate an adult-like stress response in prepubertal male rats. Neuroendocrinology 79, 125e132. Romeo, R.D., Lee, S.J., McEwen, B.S., 2004b. Differential stress reactivity in intact and ovariectomized prepubertal and adult female rats. Neuroendocrinology 80, 387e393. Romeo, R.D., McEwen, B.S., 2006. Stress and the adolescent brain. Annals of the New York Academy of Sciences 1094, 202e214. Russo, S.J., Murrough, J.W., Han, M.-H.H., Charney, D.S., Nestler, E.J., 2012. Neurobiology of resilience. Nature Neuroscience 15, 1475e1484. Ruta, L., Ingudomnukul, E., Taylor, K., Chakrabarti, B., Baron-Cohen, S., 2011. Increased serum androstenedione in adults with autism spectrum conditions. Psychoneuroendocrinology 36, 1154e1163. Scherf, K.S., Smyth, J.M., Delgado, M.R., 2013. The amygdala: an agent of change in adolescent neural networks. Hormones and Behavior 64, 298e313. Schneider, M.L., Moore, C.F., Kraemer, G.W., Roberts, A.D., DeJesus, O.T., 2002. The impact of prenatal stress, fetal alcohol exposure, or both on development: perspectives from a primate model. Psychoneuroendocrinology 27, 285e298. Schulz, K.M., Menard, T.A., Smith, D.A., Albers, H.E., Sisk, C.L., 2006. Testicular hormone exposure during adolescence organizes flank-marking behavior and vasopressin receptor binding in the lateral septum. Hormones and Behavior 50, 477e483. Schulz, K.M., Molenda-Figueira, H.A., Sisk, C.L., 2009. Back to the future: the organizational-activational hypothesis adapted to puberty and adolescence. Hormones and Behavior 55, 597e604. Schulz, K.M., Richardson, H.N., Zehr, J.L., Osetek, A.J., Menard, T.A., Sisk, C.L., 2004. Gonadal hormones masculinize and defeminize reproductive behaviors during puberty in the male Syrian hamster. Hormones and Behavior 45, 242e249. Schulz, K.M., Sisk, C.L., 2006. Pubertal hormones, the adolescent brain, and the maturation of social behaviors: lessons from the Syrian hamster. Molecular and Cellular Endocrinology 254e255, 120e126. Seeman, T.E., Singer, B., Wilkinson, C.W., McEwen, B., 2001. Gender differences in age-related changes in HPA axis reactivity. Psychoneuroendocrinology 26, 225e240. Shanmugan, S., Epperson, C.N., 2014. Estrogen and the prefrontal cortex: towards a new understanding of estrogen’s effects on executive functions in the menopause transition. Human Brain Mapping 35, 847e865. Sherer, M.L., Posillico, C.K., Schwarz, J.M., 2017. An examination of changes in maternal neuroimmune function during pregnancy and the postpartum period. Brain, Behavior, and Immunity 66, 201e209. Spindler, H., Elklit, A., Christiansen, D., 2010. Risk factors for posttraumatic stress disorder following an industrial disaster in a residential area: a note on the origin of observed gender differences. Gender Medicine 7, 156e165. Thomas, J.L., Wilk, J.E., Riviere, L.A., McGurk, D., Castro, C.A., Hoge, C.W., 2010. Prevalence of mental health problems and functional impairment among active component and National Guard soldiers 3 and 12 months following combat in Iraq. Archives of General Psychiatry 67, 614e623. Toledo-Rodriguez, M., Sandi, C., 2011. Stress during adolescence increases novelty seeking and risk-taking behavior in male and female rats. Frontiers in Behavioral Neuroscience 5, 17. Tolin, D.F., Foa, E.B., 2006. Sex differences in trauma and posttraumatic stress disorder: a quantitative review of 25 years of research. Psychological Bulletin 132, 959e992.

94

6. Sex differences in the programming of stress resilience

Tzeng, W.-Y.Y., Wu, H.-H.H., Wang, C.-Y.Y., Chen, J.-C.C., Yu, L., Cherng, C.G., 2016. Sex differences in stress and group housing effects on the number of newly proliferated cells and neuroblasts in middle-aged dentate gyrus. Frontiers in Behavioral Neuroscience 10, 249. Van Os, J., Selten, J.P., 1998. Prenatal exposure to maternal stress and subsequent schizophrenia. The May 1940 invasion of The Netherlands. British Journal of Psychiatry 172, 324e326. Vesga-López, O., Blanco, C., Keyes, K., Olfson, M., Grant, B.F., Hasin, D.S., 2008. Psychiatric disorders in pregnant and postpartum women in the United States. Archives of General Psychiatry 65, 805e815. Wade, T.J., Cairney, J., Pevalin, D.J., 2002. Emergence of gender differences in depression during adolescence: national panel results from three countries. Journal of the American Academy of Child and Adolescent Psychiatry 41, 190e198. Weathington, J.M., Arnold, A.R., Cooke, B.M., 2012. Juvenile social subjugation induces a sex-specific pattern of anxiety and depression-like behaviors in adult rats. Hormones and Behavior 61, 91e99. Wong, J., Brummelte, S., Galea, L., 2011. Elevated corticosterone levels during the first postpartum period influence subsequent pregnancy outcomes and behaviours of the dam. Journal of Neuroendocrinology 23, 1156e1165. Woolley, C.S., 2007. Acute effects of estrogen on neuronal physiology. Annual Review of Pharmacology and Toxicology 47, 657e680. Zisook, S., Rush, A.J., Lesser, I., Wisniewski, S.R., Trivedi, M., Husain, M.M., Balasubramani, G.K., Alpert, J.E., Fava, M., 2007. Preadult onset vs. adult onset of major depressive disorder: a replication study. Acta Psychiatrica Scandinavica 115, 196e205.

C H A P T E R

7

Active resilience in response to traumatic stress Gal Richter-Levin1, 2, 3, Iris Müller4, 5, Kuldeep Tripathi2, Oliver Stork4, 6 1

Department of Psychology, University of Haifa, Haifa, Israel; 2Sagol Department of Neurobiology, University of Haifa, Haifa, Israel; 3The Integrated Brain and Behavior Research Center (IBBR), University of Haifa, Haifa, Israel; 4Department of Genetics & Molecular Neurobiology, Institute of Biology, Otto-von-Guericke University Magdeburg, Magdeburg, Germany; 5Department of Psychological Sciences, Purdue University, Indianapolis, IN, United States; 6Center for Behavioral Brain Sciences, Magdeburg, Germany

Resilienceda passive lack of effect or an active response? The response to stress is variable. Some individuals are able to overcome and manage exposure to even severe stress, whereas others will be severely affected by the same experience. It is common to say “he/she was not affected by the experience,” but in fact, two different possible scenarios could explain such lack of effect at the level of behavioral symptoms: either indeed the stress experience had no effect or the stress experience did affect the neural stress response, but it was activated in such a way that enabled coping. When considering severe stressful experiences, it is in fact unlikely that the experience had no effect. It is much more likely that there was an impact, but that the response was effective in coping with the challenge. This is what we refer to here as “active resilience.” Employing a model of posttraumatic stress disorder (PTSD) we developed, we recently found that, like in humans, some animals are significantly affected by exposure to trauma, whereas others did not exhibit any significant symptoms. Interestingly, the map of activation of brain areas in the nonaffected animals was different not only from that of the affected individuals but also from that of the control, not exposed group. These results indicate that indeed, resilience is not a passive lack of response but rather an active response that enabled coping (Ritov et al., 2016). Furthermore, the selective map of activity of resilient individuals indicated a significant contribution to activation of GABAergic interneurons, suggesting that

Stress Resilience https://doi.org/10.1016/B978-0-12-813983-7.00007-0

95

Copyright © 2020 Elsevier Inc. All rights reserved.

96

7. Active resilience in response to traumatic stress

these interneurons may play an important role in active resilience (Ritov et al., 2016). Thus, although we are certain that various neural systems in several brain areas contribute to active resilience, in this chapter we will focus on one such target molecule, glutamic acid decarboxylase (GAD).

Two isozymes of glutamic acid decarboxylase g-Aminobutyric acid (GABA) is synthesized through the decarboxylation of glutamate by two isozymes of GAD, named GAD65 and GAD67, after their respective molecular weights. These two enzymes are encoded by separate genes and underlie differential modes of regulation (Erlander et al., 1991; Kaufman et al., 1991). Although these enzymes are typically coexpressed in GABAergic neurons, total levels and ratios differ greatly between brain regions and individual cells (Sheikh et al., 1999; Heldt and Ressler, 2007). Moreover, the isozymes appear to produce GABA for different cellular purposes. Several findings suggest that GAD65 produces GABA for rapid release in an activity-dependent manner: (1) it is primarily found at the synapse (Kaufman et al., 1991), (2) saturation with the cofactor pyridoxal phosphate is low (Kaufman et al., 1991; Martin et al., 1991), and (3) membrane association to GABA vesicles is reversible (Christgau et al., 1991, 1992; Reetz et al., 1991). In contrast, GAD67 is localized in the cytoplasm and synthesizes GABA for metabolic needs of the cell (Erlander et al., 1991; Kaufman et al., 1991). However, this functional distinction is by no means strict, as the isozymes can form heteromers (Sheikh and Martin, 1996). Knockout of GAD65 indicates that GAD67 can, to some extent, produce GABA for vesicular release (Wu et al., 2007). The two isozymes display specific developmental functions, as GAD67 is critical in prenatal development (Asada et al., 1997), whereas GAD65 determines GABA levels in postnatal maturation (Stork et al., 2000; Iwai et al., 2003; Ji and Obata, 1999).

GAD genes are regulated in response to fear and stress The comparison of effects of acute and chronic stress on GAD expression (Bowers et al., 1998) suggests a highly specific and differential expression regulation of both isozymes in the amygdala, bed nucleus of the stria terminalis, dentate gyrus (DG), and CA3, as well as various hypothalamic subregions, including peri-paraventricular region, medial preoptic area, anterior hypothalamic area, and the dorsomedial hypothalamic nucleus. Widespread regulation of GAD67 has recently been reported after chronic social subordination stress (Makinson et al., 2015). It should be considered that GAD expression is highly dependent on the nature of the stressor experienced and the species-specific stress response. It was shown that expression of GAD67 is reduced in the mouse amygdala after chronic restraint stress (Gilabert-Juan et al., 2011) and in the str. lacunosum-moleculare of area CA1 as well as str. lucidum and str. radiatum of area CA3 following chronic mild stress (Gilabert-Juan et al., 2016). However, another study, in rats, found no effect of restraint but reduced GAD65 in the hippocampus and reduced GAD67 in the amygdala after chronic daily corticosterone injection (Lussier et al., 2013). GAD67 is increased in the rat amygdala after

GAD is required for resilience

97

isolation rearing, with no apparent effect on GAD65 levels (Gilabert-Juan et al., 2012), but GAD65/67 immunoreactivity is lastingly decreased throughout the rat amygdala as well as in the dorsal lateral septum following a 7-day unpredictable peripubertal stress protocol (Tzanoulinou et al., 2014; Cordero et al., 2016). Moreover, GAD65 and GAD67 were found to be differentially regulated in the dorsal hippocampus and the amygdala following conditioned fear stress (Bergado-Acosta et al., 2008; Heldt and Ressler, 2007). Following Pavlovian fear conditioning, GAD65 is transiently decreased in both regions, although with different time course (6 h after training in the hippocampus, 24 h after training in the amygdala; Bergado-Acosta et al., 2008). Moreover, GAD65 in the hippocampal subregion cornu amonis (CA)1 increases in the surrounding of brain-derived neurotrophic factor (BDNF)eenriched neurons after contextual fear conditioning (Chen et al., 2007). We could further show that GAD65 expression in the dorsal DG decreases only after controllable but not after uncontrollable stress in a two-way active avoidance paradigm, whereas its expression in the basolateral amygdala (BLA) is reduced regardless of stressor controllability (Hadad-Ophir et al., 2017). In line with this, expression of GAD67, but not of GAD65, in the hippocampus and medial prefrontal cortex (PFC) was decreased in a model of chronic unpredictable stress exposure (Banasr et al., 2017). Another study showed no change of GAD65 in the BLA following unpredictable restraint stress but a positive correlation of individual expression levels in area CA1 with performance in a spatial learning task (Ortiz et al., 2015) Thus, controlling levels of GAD65 and GAD67 expression appear to provide a mechanism for fine-tuning of inhibitory function in response to different stressful experiences and stress qualities and may explain the differential effects of mutation of these genes in different forms of stress-induced psychopathology. However, these results also portray a complex relationship between GAD65 and GAD67 expression and the impact of stress exposure.

GAD is required for resilience Work from animal experiments strongly suggests that deficiency in either GAD65 or GAD67 is detrimental to the animal’s ability to resist stress-induced pathology. Homozygous GAD67(
/
) mice die shortly after birth due to the development of cleft palate (Asada et al., 1997). In contrast, GAD65(
/
) mice are viable and display no discernible morphological alterations but have around a 50% deficit in GABA content in the adult amygdala, hippocampus, and parietal cortex (Asada et al., 1996; Kash et al., 1997; Stork et al., 2000). This results in a reduction of phasic inhibition as observed in the amygdala (Lange et al., 2014) without apparent changes in the expression of GABAA receptors or response to the GABAA receptor agonist muscimol (Kash et al., 1999). Moreover, tonic inhibition may be diminished as indicated by the analysis of cortical wedges of GAD65(
/
) mice (Walls et al., 2010). Behaviorally, GAD65(
/
) mice show increased levels of anxiety, and reduced anxiolytic effects of the benzodiazepine diazepam are observed which likely result from a deficit in GABAergic function in the BLA (Kash et al., 1999; Stork et al., 2000). Recently, we demonstrated an effect of a GAD65 promoter polymorphism on harm avoidance as an endophenotype related to anxiety behavior in women, mediated by the anterior cingulate cortex (Colic et al., 2019).

98

7. Active resilience in response to traumatic stress

Furthermore, in mice we could demonstrate the generalization of auditory fear memory to a neutral acoustic stimulus in GAD65(
/
) mice (Bergado-Acosta et al., 2008). Shaban et al. (2006) reported auditory generalization upon intensive training as well as reduced presynaptic inhibition of thalamic fibers terminating in the amygdala of mice deficient for the presynaptic GABAB(1a) receptor. And indeed, GAD65 deficiency leads to a reduced spillover of GABA to presynaptic GABAB receptors (Lange et al., 2014). Of note, the observed fear generalization of GAD65(
/
) mice and associated network activity changes occurred only during long-term memory but not during short-term memory retrieval, suggesting that GAD65 serves to prevent generalization during the consolidation process of fear memory (Bergado-Acosta et al., 2008). Moreover, it appears that both GAD65 and GAD67 support fear extinction. Other than in acquisition and retrieval, GAD67 in the amygdala is of significant importance, as a regional knockdown results in profound extinction deficit (Heldt et al., 2012). Differences in GAD67 expression level in the central amygdala and its induction in both prelimbic cortex and central amygdala are associated with enhanced contextual freezing levels and responsiveness to a corticotropin-releasing factor (CRF) receptor 1 antagonist in high-anxiety rats (Skorzewska et al., 2017). Extinction of 1-day-old fear memory results in high GAD67 levels in the PFC and low GAD65 levels in hippocampal DG and CA1, whereas increased GAD65 levels are observed in the amygdala after extinction of a 14-day-old fear memory (Sangha et al., 2012). These data demonstrate the differential recruitment of GAD65 and GAD67 with respect to memory duration as, for example, the dorsal hippocampus is involved in recently acquired memories but not in older ones (Frankland et al., 2006). Accordingly, GAD65(
/
) mice 1 day after fear learning display impaired cue-specific extinction accompanied by sustained amygdalo-hippocampal synchronization (Sangha et al., 2009). GAD65 is required for preventing behavioral overreaction to a threat situation, as GAD65(
/
) mice show an altered expression of conditioned fear, that is, increased escape attempts compared with wild types. This behavioral change occurs in the presence of apparently normal autonomous or hormonal activation and is particularly evident when high-intensity unconditioned stimulus (US) are used (Stork et al., 2003) or when training and retrieval take place in the first half of their active phase (Bergado-Acosta et al., 2014). Moreover, GAD65(
/
) mice spend less time being immobile in the forced swim test (Stork et al., 2000), underlining their generally increased psychomotor activity upon stress experience. Mapping of conditioned fear-induced neural activity patterns in GAD65(
/
) mice with c-Fos immunohistochemistry revealed an increased activation of the hippocampus, amygdala, and anterior and ventromedial hypothalamus, when compared with high freezing mutants and wild types (Bergado-Acosta et al., 2014). This is in agreement with the observation that stimulation of the ventromedial hypothalamus (Wilent et al., 2010) and the amygdala (Sajdyk and Shekhar, 2000) can trigger escape behaviors and that reducing the GABAergic tone in the ventromedial hypothalamus increases anxiety- and panic-related behavior (Bueno et al., 2007) as well as fear-potentiated startle (Santos et al., 2008). Undirected activity outbreaks are characteristic for panic disorders (Blanchard et al., 1997), and application of yohimbine, a panicogenic drug, triggers fear responses similar to those of GAD65 mutants (Blanchard et al., 1993). Thus, it is not surprising that genetic studies have found an association of the GAD1 gene, encoding GAD67, with panic disorder in human patients (Hettema et al., 2006; Weber et al., 2012). A hyperexcitable hippocampus with reduced GAD65/67 expression was also reported in a genetic mouse model of panic disorder, the

GAD65 haplodeficiency conveys stress resilience

99

TrkC transgenic mouse, which could be rescued by intrahippocampal application of tiagabine, an inhibitor of GABA reuptake (Santos et al., 2013). Strikingly, in GAD65(
/
) mice, mild stress experience elicits epileptic seizures and induces c-Fos expression particularly in the DG and CA3 region of the hippocampus (Kash et al., 1997). In accordance, we observed increased probability of recurrent epileptiform discharges in area CA3 of hippocampal slices after application of moderate concentrations of kainate (Müller et al., 2015). As suggested by others, a possible mechanism behind this uncontrolled excitability could be an impairment in GABA release under elevated network excitability (such as during stress; Choi et al., 2002; Hensch, 1998; Kash et al., 1997; Tian et al., 1999). Comparably, stress is reported to be one of the leading factors responsible for triggering seizures in epilepsy patients (Frucht et al., 2000; Haut et al., 2007; Nakken et al., 2005; Spector et al., 2000; Sperling et al., 2008). Stress is also considered an important trigger for schizophrenia (Walker and Diforio, 1997). Although the majority of genetic and postmortem findings implicate GAD67 rather than GAD65 in this disease (Curley et al., 2011; Glausier et al., 2015; Guidotti et al., 2000; Impagnatiello et al., 1998; Liu et al., 2001; Volk et al., 2000), diminished GAD65 in PFC has recently been reported in a mouse model of schizophrenia (Richetto et al., 2014). Heldt et al. (2004) reported impaired prepulse inhibition (PPI) in GAD65(
/
) mice, indicating deficits in sensorimotor gating (Ludewig et al., 2002). Likewise, a mouse strain with diminished multisensory integration also displays reduced GAD65 expression (Gogolla et al., 2014). GAD65(
/
) mice further display moderate deficits in social behavior, specifically reduced aggressive behavior in the male intruder test (Stork et al., 2000). In contrast, GAD67(þ/
) mice, in spite of only minor reduction of total brain GABA content (16% in the young adult), display pronounced disturbances in social behavior, including reduced social affiliation and aggressive behavior as well as a reduced sensitivity for both social and nonsocial odors. They also present reduced baseline levels of adrenocorticotropic hormone and corticosterone levels due to a dysregulation of CRF expression in the paraventricular nucleus (Kakizawa et al., 2016) However, in contrast to GAD65(
/
) mice, they show no evidence for alterations of anxiety-like behavior (Sandhu et al., 2014), emphasizing again the isoform specificity of GAD functions. In summary, the highly dynamic and region-specific regulation of GAD65 and GAD67 expression is required to cope with stressful situations, develop appropriate behavioral responses, and prevent the development of pathologies. The upcoming task will be to determine how the stress-induced regulation of these isozymes in defined brain regions contributes to each of these protective functions and what are the precise neuronal circuits involved.

GAD65 haplodeficiency conveys stress resilience In humans, susceptibility to develop PTSD or other stress-induced psychopathologies is largely increased by the experience of childhood adversity (Chou, 2012; Ehlert, 2013), but of all trauma-exposed individuals, only about 20%e30% will go on to develop a clinical phenotype (Zohar et al., 2008). Genetic disposition, childhood abuse, and lack of social support have been identified as critical determinants for pathology development (Yehuda and LeDoux, 2007), and animal models of juvenile stress have been developed to study some of these processes (Avital and Richter-Levin, 2005; Horovitz et al., 2012; Tsoory et al., 2008).

100

7. Active resilience in response to traumatic stress

We investigated the potential interaction of juvenile stress with GAD65 mutation in GAD65(þ/
) mice, which display a delayed maturation of the GABAergic system during adolescence but normal levels in adulthood (Müller et al., 2014; Stork et al., 2000). To this end, we applied either brief variable stress (Tsoory et al., 2008) or prolonged social isolation (Pibiri et al., 2008) to juvenile GAD65 haplodeficient mice and their wild-type littermates. Both stressors varied in intensity and duration. Given the importance of proper GABAergic functioning in response to stress (Vaiva et al., 2006, 2004) and the delayed GABAergic maturation in these mice (Stork et al., 2000), we expected increased vulnerability in GAD65(þ/
) mice to adulthood challenges such as fear conditioning. In contrast, resilience to the variable juvenile stress regimen could be observed, as haplodeficient mice displayed significantly reduced contextual generalization of fear memory following juvenile variable stress (Müller et al., 2014). No comparable genotype difference was observed concerning social affiliation and depression-like behavior in a tail suspension test, indicating that the protective effect of GAD65 haplodeficiency was specific for the behavioral fear domain (Müller et al., 2014). Moreover, we could rule out an effect on fear expression, as hyperactivity bouts were not increased in GAD65(þ/
) mice (Müller et al., 2015). This indicates that, in GAD65(þ/
) mice, adaptive processes can be triggered by brief stressful events in youth, before GABA production reaches its normal level in adulthood (Müller et al., 2015), and that this delayed maturation of the GABAergic system can somehow enable later resilience toward challenges in adulthood.

GAD65 and stress resilienceda complex picture Results obtained so far clearly indicate that GAD65 functioning is relevant to stress vulnerability and stress resilience. However, the variable results regarding stress-related alterations in the expression of GAD65 reported above, together with the differences between effects of homozygous GAD65(
/
) knockout and GAD65 haplodeficiency (GAD65(þ/
) mice), suggest that the role of GAD65 in stress vulnerability and stress resilience may differ in different brain regions and between different developmental stages. More temporal- and spatial-specific manipulations of expression are required to more accurately describe the role played by this enzyme in coping with stress. Recently, we initiated a study in which the expression of GAD65 is modulated by means of viral vectors in a temporal- and spatial-specific manner. This approach enables deciphering the role of local alterations in GAD expression at specific developmental time periods. Although the results are still accumulating, we found that reducing the expression of GAD65 in the ventral hippocampus of adult rats has a very different impact compared with a similar manipulation in the dorsal hippocampus. Reducing the expression of GAD65 in the ventral hippocampus is associated with increased anxiety, as measured in the open field test and the elevated plus maze. In contrast, reducing the expression of GAD65 in the dorsal hippocampus resulted in a significant reduction in anxiety behavior. Potentially, such local manipulation could be expected to reduce the impact of exposure to trauma and increase resilience. We have started examining this possibility by first exposing animals to juvenile trauma, then employing such viral vector manipulation in the dorsal hippocampus, and testing the impact of the trauma in adulthood, comparing between animals

References

101

that did or did not receive that manipulation. Preliminary data seem very promising, and although more experiments are required to finalize these findings, the results obtained so far strongly support the concept that GAD65 levels of expression may affect both stress vulnerability and stress resilience, depending on the brain region and the developmental stage under investigation.

Summary GABAergic malfunctions are present in various mental disorders, but further research will have to consider that GABAergic interneurons in the brain appear with an almost bewildering variety of anatomical, physiological, and neurochemical features. Different subpopulations of such interneurons are perfectly equipped for patterning the information input (dendrite targeting interneurons), controlling cellular excitability and output generation (basket cells and chandelier cells), and controlling various forms of complex network activities (Winkelmann et al., 2014). These interneurons use a variety of neuropeptide cotransmitters that themselves exert profound effects on fear, anxiety, and stress response (e.g., neuropeptide Y, cholecystokinin, Lach and de Lima, 2013; Serova et al., 2014; Sherrin et al., 2009; Hadad-Ophir et al., 2017; Banasr et al., 2017; Raza et al., 2017). The interaction of neuropeptide function with GAD65-mediated GABA synthesis still needs to be explored. Future studies with conditional mutants and acute genetic intervention tools will be able to address the observed differential GAD65 and GAD67 regulation in such selected interneuron populations and their roles in stress-induced pathogenesis and stress resilience.

Acknowledgments This work was supported by the German Research Foundation (Project STO488/6 to OS and GRL).

References Asada, H., Kawamura, Y., Maruyama, K., Kume, H., Ding, R., Ji, F.Y., Kanbara, N., Kuzume, H., Sanbo, M., Yagi, T., Obata, K., 1996. Mice lacking the 65 kDa isoform of glut amic acid decarboxylase (GAD65) maintain normal levels o f GAD67 and GABA in their brains bu t are susceptible to seizures. Biochemical and Biophysical Research Communications 895, 891e895. Asada, H., Kawamura, Y., Maruyama, K., Kume, H., Ding, R.G., Kanbara, N., Kuzume, H., Sanbo, M., Yagi, T., Obata, K., 1997. Cleft palate and decreased brain gamma-aminobutyric acid in mice lacking the 67-kDa isoform of glutamic acid decarboxylase. Proceedings of the National Academy of Sciences of the United States of America 94, 6496e6499. Avital, A., Richter-Levin, G., 2005. Exposure to juvenile stress exacerbates the behavioural consequences of exposure to stress in the adult rat. The International Journal of Neuropsychopharmacology 8 (2), 163e173. Banasr, M., Lepack, A., Fee, C., Duric, V., Maldonado-Aviles, J., DiLeone, R., Sibille, E., Duman, R.S., Sanacora, G., 2017. Characterization of GABAergic marker expression in the chronic unpredictable stress model of depression. Chronic Stress (Thousand Oaks) 1. https://doi.org/10.1177/2470547017720459. Bergado-Acosta, J.R., Müller, I., Richter-Levin, G., Stork, O., 2014. The GABA-synthetic enzyme GAD65 controls circadian activation of conditioned fear pathways. Behavioural Brain Research 260, 92e100.

102

7. Active resilience in response to traumatic stress

Bergado-Acosta, J.R., Sangha, S., Narayanan, R.T., Obata, K., Pape, H., Stork, O., 2008. Critical role of the 65-kDa isoform of glutamic acid decarboxylase in consolidation and generalization of Pavlovian fear memory. Learning and Memory 15, 163e171. Blanchard, R.J., Griebel, G., Henrie, J.A., Blanchard, D.C., 1997. Differentiation of anxiolytic and panicolytic drugs by effects on rat and mouse defense test batteries. Neuroscience and Biobehavioral Reviews 21, 783e789. Blanchard, R.J., Taukulis, H.K., Rodgers, R.J., Magee, L.K., Blanchard, D.C., 1993. Yohimbine potentiates active defensive responses to threatening stimuli in Swiss-Webster mice. Pharmacology Biochemistry and Behavior 44, 673e681. Bowers, G., Cullinan, W.E., Herman, J.P., 1998. Region-specific regulation of glutamic acid decarboxylase (GAD) mRNA expression in central stress circuits. Journal of Neuroscience 18, 5938e5947. Bueno, C.H., Zangrossi, H., Viana, M.D.B., 2007. GABA/benzodiazepine receptors in the ventromedial hypothalamic nucleus regulate both anxiety and panic-related defensive responses in the elevated T-maze. Brain Research Bulletin 74, 134e141. Chen, J., Kitanishi, T., Ikeda, T., Matsuki, N., Yamada, M.K., 2007. Contextual learning induces an increase in the number of hippocampal CA1 neurons expressing high levels of BDNF. Neurobiology of Learning and Memory 88, 409e415. Choi, S.-Y., Morales, B., Lee, H.-K., Kirkwood, A., 2002. Absence of long-term depression in the visual cortex of glutamic Acid decarboxylase-65 knock-out mice. Journal of Neuroscience 22, 5271e5276. Chou, K.L., 2012. Childhood sexual abuse and psychiatric disorders in middle-aged and older adults: evidence from the 2007 Adult Psychiatric Morbidity Survey. Journal of Clinical Psychiatry 73, e1365ee1371. Christgau, S., Aanstoot, H.J., Schierbeck, H., Begley, K., Tullin, S., Hejnaes, K., Baekkeskov, S., 1992. Membrane anchoring of the autoantigen GAD65 to microvesicles in pancreatic beta-cells by palmitoylation in the NH2terminal domain. The Journal of Cell Biology 118, 309e320. Christgau, S., Schierbeck, H., Aanstoot, H.J., Aagaard, L., Begley, K., Kofod, H., Hejnaes, K., Baekkeskov, S., 1991. Pancreatic beta cells express two autoantigenic forms of glutamic acid decarboxylase, a 65-kDa hydrophilic form and a 64-kDa amphiphilic form which can be both membrane-bound and soluble. Journal of Biological Chemistry 266, 23516. Colic, L., Li, M., Ramona Demenescu, L., Li, S., Müller, I., Richter, A., Seidenbecher, C.I., Speck, O., Schott, B.H., Stork, O., Walter, M., 2019. GAD65 promoter polymorphism rs2236418 modulates harm avoidance in women via inhibition/excitation balance in the rostral ACC. The Journal of Neuroscience 38 (22), 5067e5077. Cordero, M.I., Just, N., Poirier, G.L., Sandi, C., 2016. Effects of paternal and peripubertal stress on aggression, anxiety, and metabolic alterations in the lateral septum. European Neuropsychopharmacology 26 (2), 357e367. Curley, A.A., Arion, D., Volk, D.W., Asafu-Adjei, J.K., Sampson, A.R., Fish, K.N., Lewis, D.A., 2011. Cortical deficits of glutamic acid decarboxylase 67 expression in schizophrenia: clinical, protein, and cell type-specific features. American Journal of Psychiatry 168, 921e929. Ehlert, U., 2013. Enduring psychobiological effects of childhood adversity. Psychoneuroendocrinology 38, 1850e1857. Erlander, M.G., Tillakaratne, N.J., Feldblum, S., Patel, N., Tobin, A.J., 1991. Two genes encode distinct glutamate decarboxylases. Neuron 7, 91e100. Frankland, P.W., Ding, H.-K., Takahashi, E., Suzuki, A., Kida, S., Silva, A.J., 2006. Stability of recent and remote contextual fear memory. Learning and Memory 13, 451e457. Frucht, M.M., Quigg, M., Schwaner, C., Fountain, N.B., 2000. Distribution of seizure precipitants among epilepsy syndromes. Epilepsia 41, 1534e1539. Gilabert-Juan, J., Castillo-Gomez, E., Pérez-Rando, M., Moltó, M.D., Nacher, J., 2011. Chronic stress induces changes in the structure of interneurons and in the expression of molecules related to neuronal structural plasticity and inhibitory neurotransmission in the amygdala of adult mice. Experimental Neurology 232, 33e40. Gilabert-Juan, J., Moltó, M.D., Nacher, J., 2012. Post-weaning social isolation rearing influences the expression of molecules related to inhibitory neurotransmission and structural plasticity in the amygdala of adult rats. Brain Research 1448, 129e136. Gilabert-Juan, J., Bueno-Fernandez, C., Castillo-Gomez, E., Nacher, J., 2016. Reduced interneuronal dendritic arborization in CA1 but not in CA3 region of mice subjected to chronic mild stress. Brain and Behavior 7 (2), e00534. Glausier, J.R., Kimoto, S., Fish, K.N., Lewis, D.A., 2015. Lower glutamic acid decarboxylase 65-kDa isoform messenger RNA and protein levels in the prefrontal cortex in schizoaffective disorder but not schizophrenia. Biological Psychiatry 77 (2), 167e176.

References

103

Gogolla, N., Takesian, A.E., Feng, G., Fagiolini, M., Hensch, T.K., 2014. Sensory integration in mouse insular cortex reflects GABA circuit maturation. Neuron 83, 894e905. Guidotti, A., Auta, J., Davis, J.M., Gerevini, V.D., Dwivedi, Y., Grayson, D.R., Impagnatiello, F., Pandey, G., 2000. Decrease in reelin and glutamic acid decarboxylase 67 (GAD 67 ) expression in schizophrenia and bipolar disorder: a post mortem brain study. Archives of General Psychiatry 57, 1061e1069. Hadad-Ophir, O., Ardi, Z., Brande-Eilat, N., Kehat, O., Anunu, R., Richter-Levin, G., 2017. Exposure to prolonged controllable or uncontrollable stress affects GABAergic function in sub-regions of the hippocampus and the amygdala. Neurobiology of Learning and Memory 138, 271e280. Haut, S.R., Hall, C.B., Masur, J., Lipton, R.B., 2007. Seizure occurrence: precipitants and predictions. Neurology 69, 1905e1910. Heldt, S.A., Green, A., Ressler, K.J., 2004. Prepulse inhibition deficits in GAD65 knockout mice and the effect of antipsychotic treatment. Neuropsychopharmacology 29, 1610e1619. Heldt, S.A., Mou, L., Ressler, K.J., 2012. In vivo knockdown of GAD67 in the amygdala disrupts fear extinction and the anxiolytic-like effect of diazepam in mice. Translational Psychiatry 2, e181. Heldt, S.A., Ressler, K.J., 2007. Training-induced changes in the expression of GABAA-associated genes in the amygdala after the acquisition and extinction of Pavlovian fear. European Journal of Neuroscience 26, 3631e3644. Hensch, T.K., 1998. Local GABA circuit control of experience-dependent plasticity in developing visual cortex. Science 282, 1504e1508. Hettema, J.M., An, S.S., Neale, M.C., Bukszar, J., van den Oord, E.J., Kendler, K.S., Chen, X., 2006. Association between glutamic acid decarboxylase genes and anxiety disorders, major depression, and neuroticism. Molecular Psychiatry 11, 752e762. Horovitz, O., Tsoory, M.M., Hall, J., Jacobson-Pick, S., Richter-Levin, G., 2012. Post-weaning to pre-pubertal (‘juvenile’) stress: a model of induced predisposition to stress-related disorders. Neuroendocrinology 95, 56e64. Impagnatiello, F., Guidotti, A.R., Pesold, C., Dwivedi, Y., Caruncho, H., Pisu, M.G., Uzunov, D.P., Smalheiser, N.R., Davis, J.M., Pandey, G.N., Pappas, G.D., Tueting, P., Sharma, R.P., Costa, E., 1998. A decrease of reelin expression as a putative vulnerability factor in schizophrenia. Proceedings of the National Academy of Sciences of the United States of America 95, 15718e15723. Iwai, Y., Fagiolini, M., Obata, K., Hensch, T.K., 2003. Rapid critical period induction by tonic inhibition in visual cortex. Journal of Neuroscience 23, 6695e6702. Ji, F., Obata, K., 1999. Development of the GABA system in organotypic culture of hippocampal and cerebellar slices from a 67-kDa isoform of glutamic acid decarboxylase (GAD67)-deficient mice. Neuroscience Research 33, 233e237. Kakizawa, K., Watanabe, M., Mutoh, H., Okawa, Y., Yamashita, M., Yanagawa, Y., Itoi, K., Suda, T., Oki, Y., Fukuda, A., 2016. A novel GABA-mediated corticotropin-releasing hormone secretory mechanism in the median eminence. Science Advances 2 (8), e1501723. Kash, S.F., Johnson, R.S., Tecott, L.H., Noebels, J.L., Mayfield, R.D., Hanahan, D., Baekkeskov, S., 1997. Epilepsy in mice deficient in the 65-kDa isoform of glutamic acid decarboxylase. Proceedings of the National Academy of Sciences of the United States of America 94, 14060e14065. Kash, S.F., Tecott, L.H., Hodge, C., Baekkeskov, S., 1999. Increased anxiety and altered responses to anxiolytics in mice deficient in the 65-kDa isoform of glutamic acid decarboxylase. Proceedings of the National Academy of Sciences of the United States of America 96, 1698e1703. Kaufman, D.L., Houser, C.R., Tobin, A.J., 1991. Two forms of the gamma-aminobutyric acid synthetic enzyme glutamate decarboxylase have distinct intraneuronal distributions and cofactor interactions. Journal of Neurochemistry 56, 720e723. Lach, G., de Lima, T.C., 2013. Role of NPY Y1 receptor on acquisition, consolidation and extinction on contextual fear conditioning: dissociation between anxiety, locomotion and non-emotional memory behavior. Neurobiology of Learning and Memory 103, 26e33. Lange, M.D., Jüngling, K., Paulukat, L., Vieler, M., Gaburro, S., Sosulina, L., Blaesse, P., Sreepathi, H.K., Ferraguti, F., Pape, H.-C., 2014. Glutamic acid decarboxylase 65: a link between GABAergic synaptic plasticity in the lateral amygdala and conditioned fear generalization. Neuropsychopharmacology 39, 2211e2220. Liu, W.S., Pesold, C., Rodriguez, M. a, Carboni, G., Auta, J., Lacor, P., Larson, J., Condie, B.G., Guidotti, A., Costa, E., 2001. Down-regulation of dendritic spine and glutamic acid decarboxylase 67 expressions in the reelin haploinsufficient heterozygous reeler mouse. Proceedings of the National Academy of Sciences of the United States of America 98, 3477e3482.

104

7. Active resilience in response to traumatic stress

Ludewig, K., Geyer, M.A., Etzensberger, M., Vollenweider, F.X., 2002. Stability of the acoustic startle reflex, prepulse inhibition, and habituation in schizophrenia. Schizophrenia Research 55, 129e137. Lussier, A.L., Romay-Tallón, R., Caruncho, H.J., Kalynchuk, L.E., 2013. Altered GABAergic and glutamatergic activity within the rat hippocampus and amygdala in rats subjected to repeated corticosterone administration but not restraint stress. Neuroscience 231, 38e48. Makinson, R., Lundgren, K.H., Seroogy, K.B., Herman, J.P., 2015. Chronic social subordination stress modulates glutamic acid decarboxylase (GAD) 67 mRNA expression in central stress circuits. Physiology and Behavior 146, 7e15. Martin, D.L., Martin, S.B., Wu, S.J., Espinas, N., 1991. Regulatory properties of brain glutamate decarboxylase (GAD): the apoenzyme of GAD is present principally as the smaller of two molecular forms of GAD in brain. Journal of Neuroscience 11, 2725e2731. Müller, I., Obata, K., Richter-Levin, G., Stork, O., 2014. GAD65 haplodeficiency conveys resilience in animal models of stress-induced psychopathology. Frontiers in Behavioral Neuroscience 8, 265. Müller, I., Çalıskan, G., Stork, O., 2015. The GAD65 knock out mouse - a model for GABAergic processes in fear- and stress-induced psychopathology. Genes, Brain and Behavior 14 (1), 37e45. Nakken, K.O., Solaas, M.H., Kjeldsen, M.J., Friis, M.L., Pellock, J.M., Corey, L.A., 2005. Which seizure-precipitating factors do patients with epilepsy most frequently report? Epilepsy and Behavior 6, 85e89. Ortiz, J.B., Taylor, S.B., Hoffman, A.N., Campbell, A.N., Lucas, L.R., Conrad, C.D., 2015. Sex-specific impairment and recovery of spatial learning following the end of chronic unpredictable restraint stress: potential relevance of limbic GAD. Behavioural Brain Research 282, 176e184. Pibiri, F., Nelson, M., Guidotti, A., Costa, E., Pinna, G., 2008. Decreased corticolimbic allopregnanolone expression during social isolation enhances contextual fear: a model relevant for posttraumatic stress disorder. Proceedings of the National Academy of Sciences of the United States of America 105, 5567e5572. Raza, S.A., Albrecht, A., Çalıskan, G., Müller, B., Demiray, Y.E., Ludewig, S., Meis, S., Faber, N., Hartig, R., Schraven, B., Lessmann, V., Schwegler, H., Stork, O., 2017. HIPP neurons in the dentate gyrus mediate the cholinergic modulation of background context memory salience. Nature Communications 8 (1), 189. Reetz, a, Solimena, M., Matteoli, M., Folli, F., Takei, K., De Camilli, P., 1991. GABA and pancreatic beta-cells: colocalization of glutamic acid decarboxylase (GAD) and GABA with synaptic-like microvesicles suggests their role in GABA storage and secretion. The EMBO Journal 10, 1275e1284. Richetto, J., Calabrese, F., Riva, M.A., Meyer, U., 2014. Prenatal immune activation induces maturation-dependent alterations in the prefrontal GABAergic transcriptome. Schizophrenia Bulletin 40, 351e361. Ritov, G., Boltyansky, B., Richter-Levin, G., 2016. A novel approach to PTSD modeling in rats reveals alternating patterns of limbic activity in different types of stress reaction. Molecular Psychiatry 21 (5), 630e641. Sajdyk, T.J., Shekhar, A., 2000. Sodium lactate elicits anxiety in rats after repeated GABA receptor blockade in the basolateral amygdala. European Journal of Pharmacology 394, 265e273. Sandhu, K.V., Lang, D., Müller, B., Nullmeier, S., Yanagawa, Y., Schwegler, H., Stork, O., 2014. Glutamic acid decarboxylase 67haplodeficiency impairs social behavior in mice. Genes, Brain and Behavior 13, 439e450. Sangha, S., Ilenseer, J., Sosulina, L., Lesting, J., Pape, H.-C., 2012. Differential regulation of glutamic acid decarboxylase gene expression after extinction of a recent memory vs. intermediate memory. Learning and Memory 19, 194e200. Sangha, S., Narayanan, R.T., Bergado-Acosta, J.R., Stork, O., Seidenbecher, T., Pape, H.-C., 2009. Deficiency of the 65 kDa isoform of glutamic acid decarboxylase impairs extinction of cued but not contextual fear memory. Journal of Neuroscience 29, 15713e15720. Santos, J.M., Macedo, C.E., Brandão, M.L., 2008. Gabaergic mechanisms of hypothalamic nuclei in the expression of conditioned fear. Neurobiology of Learning and Memory 90, 560e568. Santos, M., D’Amico, D., Spadoni, O., Amador-Arjona, A., Stork, O., Dierssen, M., 2013. Hippocampal hyperexcitability underlies enhanced fear memories in TgNTRK3, a panic disorder mouse model. Journal of Neuroscience 33, 15259e15271. Serova, L.I., Laukova, M., Alaluf, L.G., Pucillo, L., Sabban, E.L., 2014. Intranasal neuropeptide Y reverses anxiety and depressive-like behavior impaired by single prolonged stress PTSD model. European Neuropsychopharmacology 24, 142e147. Shaban, H., Humeau, Y., Herry, C., Cassasus, G., Shigemoto, R., Ciocchi, S., Barbieri, S., van der Putten, H., Kaupmann, K., Bettler, B., Lüthi, A., 2006. Generalization of amygdala LTP and conditioned fear in the absence of presynaptic inhibition. Nature Neuroscience 9, 1028e1035.

References

105

Sheikh, S.N., Martin, D.L., 1996. Heteromers of glutamate decarboxylase isoforms occur in rat cerebellum. Journal of Neurochemistry 66, 2082e2090. Sheikh, S.N., Martin, S.B., Martin, D.L., 1999. Regional distribution and relative amounts of glutamate decarboxylase isoforms in rat and mouse brain. Neurochemistry International 35, 73e80. Sherrin, T., Todorovic, C., Zeyda, T., Tan, C.H., Wong, P.T., Zhu, Y.Z., Spiess, J., 2009. Chronic stimulation of corticotropin-releasing factor receptor 1 enhances the anxiogenic response of the cholecystokinin system. Molecular Psychiatry 14, 291e307. Skorzewska, A., Wislowska-Stanek, A., Lehner, M., Turzynska, D., Sobolewska, A., Krzascik, P., Plaznik, A., 2017. Corticotropin releasing factor receptor 1 antagonist differentially inhibits freezing behavior and changes gamma-aminobutyric acidergic activity in the amygdala in low- and high-anxiety rats. Journal of Physiology and Pharmacology 68 (1), 35e46. Spector, S., Cull, C., Goldstein, L.H., 2000. Seizure precipitants and perceived self-control of seizures in adults with poorly-controlled epilepsy. Epilepsy Research 38, 207e216. Sperling, M.R., Schilling, C.A., Glosser, D., Tracy, J.I., Asadi-Pooya, A.A., 2008. Self-perception of seizure precipitants and their relation to anxiety level, depression, and health locus of control in epilepsy. Seizure 17, 302e307. Stork, O., Ji, F.Y., Kaneko, K., Stork, S., Yoshinobu, Y., Moriya, T., Shibata, S., Obata, K., 2000. Postnatal development of a GABA deficit and disturbance of neural functions in mice lacking GAD65. Brain Research 865, 45e58. Stork, O., Yamanaka, H., Stork, S., Kume, N., Obata, K., 2003. Altered conditioned fear behavior in glutamate decarboxylase 65 null mutant mice. Genes, Brain and Behavior 2, 65e70. Tian, N., Petersen, C., Kash, S., Baekkeskov, S., Copenhagen, D., Nicoll, R., 1999. The role of the synthetic enzyme GAD65 in the control of neuronal gamma-aminobutyric acid release. Proceedings of the National Academy of Sciences of the United States of America 96, 12911e12916. Tsoory, M., Guterman, A., Richter-Levin, G., 2008. Exposure to stressors during juvenility disrupts developmentrelated alterations in the PSA-NCAM to NCAM expression ratio: potential relevance for mood and anxiety disorders. Neuropsychopharmacology 33, 378e393. Tzanoulinou, S., García-Mompó, C., Castillo-Gómez, E., Veenit, V., Nacher, J., Sandi, C., 2014. Long-term behavioral programming induced by peripuberty stress in rats is accompanied by GABAergic-related alterations in the Amygdala. PLoS One 9, e94666. Vaiva, G., Boss, V., Ducrocq, F., Fontaine, M., Devos, P., Brunet, A., Laffargue, P., Goudemand, M., Thomas, P., 2006. Relationship between posttrauma GABA plasma levels and PTSD at 1-year follow-up. American Journal of Psychiatry 163, 1446e1448. Vaiva, G., Thomas, P., Ducrocq, F., Fontaine, M., Boss, V., Devos, P., Rascle, C., Cottencin, O., Brunet, A., Laffargue, P., Goudemand, M., 2004. Low posttrauma GABA plasma levels as a predictive factor in the development of acute posttraumatic stress disorder. Biological Psychiatry 55, 250e254. Volk, D.W., Austin, M.C., Pierri, J.N., Sampson, A.R., Lewis, D.A., 2000. Decreased glutamic acid decarboxylase 67 messenger RNA expression in a subset of prefrontal cortical gamma-aminobutyric acid neurons in subjects with schizophrenia. Archives of General Psychiatry 57, 237e245. Walker, E.F., Diforio, D., 1997. Schizophrenia: a neural diathesis-stress model. Psychological Review 104, 667e685. Walls, A.B., Nilsen, L.H., Eyjolfsson, E.M., Vestergaard, H.T., Hansen, S.L., Schousboe, A., Sonnewald, U., Waagepetersen, H.S., 2010. GAD65 is essential for synthesis of GABA destined for tonic inhibition regulating epileptiform activity. Journal of Neurochemistry 115, 1398e1408. Weber, H., Scholz, C.J., Domschke, K., Baumann, C., Klauke, B., Jacob, C.P., Maier, W., Fritze, J., Bandelow, B., Zwanzger, P.M., Lang, T., Fehm, L., Ströhle, A., Hamm, A., Gerlach, A.L., Alpers, G.W., Kircher, T., Wittchen, H.U., Arolt, V., Pauli, P., Deckert, J., Reif, A., 2012. Gender differences in associations of glutamate decarboxylase 1 gene (GAD1) variants with panic disorder. PLoS One 7, e37651. Wilent, W.B., Oh, M.Y., Buetefisch, C.M., Bailes, J.E., Cantella, D., Angle, C., Whiting, D.M., 2010. Induction of panic attack by stimulation of the ventromedial hypothalamus. Journal of Neurosurgery 112, 1295e1298. Winkelmann, A., Maggio, N., Eller, J., Caliskan, G., Semtner, M., Häussler, U., Jüttner, R., Dugladze, T., Smolinsky, B., Kowalczyk, S., Chronowska, E., Schwarz, G., Rathjen, F.G., Rechavi, G., Haas, C.A., Kulik, A., Gloveli, T., Heinemann, U., Meier, J.C., 2014. Changes in neural network homeostasis trigger neuropsychiatric symptoms. Journal of Clinical Investigation 124, 696e711.

106

7. Active resilience in response to traumatic stress

Wu, H., Jin, Y., Buddhala, C., Osterhaus, G., Cohen, E., Jin, H., Wei, J., Davis, K., Obata, K., Wu, J.-Y., 2007. Role of glutamate decarboxylase (GAD) isoform, GAD65, in GABA synthesis and transport into synaptic vesiclesEvidence from GAD65-knockout mice studies. Brain Research 1154, 80e83. Yehuda, R., LeDoux, J., 2007. Response variation following trauma: a translational neuroscience approach to understanding PTSD. Neuron 56, 19e32. Zohar, J., Juven-Wetzler, A., Myers, V., Fostick, L., 2008. Post-traumatic stress disorder: facts and fiction. Current Opinion in Psychiatry 21, 74e77.

C H A P T E R

8

Rhythms of stress resilience Francesca Spiga, Stafford L. Lightman Bristol Medical School: Translational Health Sciences, University of Bristol, Bristol, United Kingdom

Hypothalamic-pituitary-adrenal axis rhythms Glucocorticoids (cortisol in humans and corticosterone in rodents; GCs) are vital hormones that regulate many physiological functions, including glucose, fat, and protein metabolism (Cherrington, 1999; Macfarlane et al., 2008; Munck et al., 1984). In addition, GCs exert antiinflammatory and immunosuppressive actions and can affect mood and cognitive function (Chrousos, 1995; de Kloet, 2000; McEwen, 2007). Circulating levels of GC are regulated by the activity of the hypothalamic-pituitary-adrenal (HPA) axis (Fig. 8.1A). The activity of the HPA axis increases when the organism is exposed to stress. The HPA axis is also active under basal (i.e., unstressed) conditions, and the release of GCs hormones is characterized by a circadian rhythm (Fig. 8.1B), with higher levels of hormone during the active phase (night in rodents, day in human). The circadian variation in GC levels over the 24-h cycle is not made up of a smooth change in hormone levels but is in fact characterized by a rapid ultradian, pulsatile pattern of hormone secretion, with a periodicity of approximately 1 h in the rat (Fig. 8.1B). Ultradian GC rhythm has been reported in numerous species, including rat (Jasper and Engeland, 1991; Windle et al., 1998b), rhesus monkey (Holaday et al., 1977; Tapp et al., 1984), sheep (Fulkerson, 1978), and human (Henley et al., 2009; Lewis et al., 2005; Weitzman et al., 1971). Circadian and ultradian rhythms of GCs are important factors in determining the behavioral, neuroendocrine, and genomic response to stressors. A number of studies have shown that disruption of these rhythms occurs in a number of physiological and pathological conditions, including aging and chronic inflammatory disease (reviewed in Spiga et al., 2014). Importantly, changes in GC rhythms are also associated with changes in the GCs response to stressors. In this chapter, we will address the importance of GC circadian and ultradian rhythms for stress resilience; for the sake of clarity, we will describe findings from studies in rodent, unless differently specified.

Stress Resilience https://doi.org/10.1016/B978-0-12-813983-7.00008-2

107

Copyright © 2020 Elsevier Inc. All rights reserved.

108

8. Rhythms of stress resilience

FIGURE 8.1 (A) The hypothalamic-pituitary-adrenal (HPA) axis and (B) circadian and ultradian rhythm of glucocorticoid secretion in the rat.Reproduced with permission from Spiga et al., 2014.

Circadian rhythm and stress response The circadian rhythm of the HPA axis is regulated by light inputs via the oscillatory activity of the biological master clock, the suprachiasmatic nucleus (Spiga et al., 2014). Cells within the SCN possess a circadian activity regulated by an oscillating transcriptional network of clock genes. This is composed by transcriptional, translational, and posttranslational feedback loops in which heterodimers of the activator proteins CLOCK and BMAL (encoded by the Clock and Bmal1 genes, respectively) regulate the expression of the repressors proteins CRYs (encoded by the Cry1 and Cry2 genes) and PERs (encoded by the Per1, Per2, Per3 genes). CRYs and PERs in turn translocate back into the nucleus and repress the activities of the BMAL-CLOCK complex, thus establishing circadian rhythmicity in gene expression (Lowrey and Takahashi, 2011; Panda et al., 2002). It has been known for long time that the responsiveness of the HPA axis to stress depends on the time of day of exposure to the stress (Atkinson et al., 2006; Dunn et al., 1972; Gallant and Brownie, 1979; Gibbs, 1970; Kant et al., 1986; Torrellas et al., 1981; Zimmermann and Critchlow, 1967). However, the mechanisms underlying this time-dependent responsiveness are still not clear. Several studies have shown that disruptions of the circadian clock result in altered basal HPA axis activity and GC concentration; however, only few have investigated changes in HPA axis responsiveness to stress in clock gene mutant mice. Basal GC concentration is elevated in Cry mutant mice (Barclay et al., 2013; Lamia et al., 2011; Leliavski et al., 2014; Turek et al., 2005; Yang et al., 2009), whereas mutations of Bmal1, Clock, and Per are associated with decreased hormone concentration (Barclay et al., 2013; Lamia et al., 2011; Leliavski et al., 2014; Turek et al., 2005; Yang et al., 2009). However, mutation of Cry and Bmal1 is associated with reduced responsiveness to stress (Barclay et al., 2013; Lamia et al., 2011; Zhang et al., 2011), whereas mutation of Per is associated with enhanced response to stress (Barclay et al., 2013; Lamia et al., 2011; Zhang et al., 2011).

The importance of pulsatility for hormonal and behavioral response to stress

109

The effects of clock gene mutations on GC concentrations can occur at different levels of the HPA axis. For example, deletion of Bmal1 leads to decreased adrenal sensitivity to ACTH during the active phase (Bartlang et al., 2012; Engeland et al., 1977; Leliavski et al., 2014; Oster et al., 2006), resulting in decreased GC response to stress. Furthermore, CRY protein can bind to the GC receptor, inhibiting its activity and, therefore, the effects of GCs. Thus, hypersecretion of GC in Cry mutant mice is due to impaired GR-mediated negative feedback inhibition in the brain and pituitary (Lamia et al., 2011). GC responsiveness to stress not only depends on the time of day but also on the nature of the stressor. For example, exposure to physical stressors such as hemorrhage (Lilly et al., 2000) or hypoglycemia (Kalsbeek et al., 2003) during the active phase results in an increase in circulating GCs that is greater than when exposure to the stressors occurs during the inactive phase. In contrast, exposure to psychological stressors such as novelty (Buijs et al., 1997), restraint (Bradbury et al., 1991), inflammation (Mathias et al., 2000), foot shock and immobilization (Retana-Marquez et al., 2003), shaking stress (Bernatova et al., 2002), as well as procedures that are routine in animal house maintenance (i.e., handling, cage changing, grouping) (Gattermann and Weinandy, 1996) during the inactive phase results in a stronger increase in GC release than during the active phase.

The importance of pulsatility for hormonal and behavioral response to stress A number of studies have investigated the functional interaction between GC pulsatility and the response to stress. In these studies, when rats were exposed to noise stress during different phases of their ultradian corticosterone secretory profile (Sarabdjitsingh et al., 2010; Windle et al., 1998a). The timing of exposure to the stressor, relative to the phase of the ultradian rhythm, was crucial in determining the magnitude of the corticosterone response. Indeed, in this study, it was found that the corticosterone response is considerably greater when the stressor is applied during the rising phase of a corticosterone pulse than during the falling phase (Fig. 8.2). These findings suggest a facilitated stress response during the rising phase and/or an inhibitory effect during the falling phase and indicate the existence of a dynamic interaction between basal GC pulsatility and the ability of an animal to mount an optimal hormonal stress response. To understand in more detail the interaction between pulsatility and stress responsiveness, a mathematical modeling approach has also been used (Rankin et al., 2012). In addition to explaining earlier observations that the magnitude of the stress response depends on the timing of the stress, this model has also shown that an external stress can act as a resetting mechanism to the phase of the endogenous ultradian rhythm; that is, depending on the timing of the stress, the phase of the ultradian rhythm can either be advanced or delayed. The mechanism underlying the differential HPA axis response to stress in relationship to the ultradian corticosterone rhythm has been further investigated in adrenalectomized rats in which the endogenous hormone was replaced with an intravenous infusion of hourly pulses of corticosterone (Sarabdjitsingh et al., 2010). Consistent with previous studies (Windle et al., 1998b), exposure of these rats to noise stress results in an increase in ACTH that is more

110

8. Rhythms of stress resilience

(A)

(B)

FIGURE 8.2 The phase of the ultradian corticosterone rhythm is important for the amplitude of the stress response. Rats were exposed to noise stress (10 min, 114 dB). Rats were stressed during the rising phase (A) or the falling phase (B) of an endogenous corticosterone pulse. rats exposed to stress during the rising phase show much greater corticosterone responses than animals stressed during the falling phase. Reproduced with permission from Spiga et al., 2014.

pronounced when the stressor is applied during the rising phase than during the falling phase of the corticosterone pulse. The same study also shows that the differential ACTH response to noise stress, relative to the phase of corticosterone pulse, was also associated with a different behavioral response to the stressor. Indeed, the behavioral response to the noise stress was higher in rats stressed during the rising phase than the falling phase of the corticosterone pulse. These findings are consistent with earlier behavioral studies showing that the corticosterone response of rats exposed to a male intruder is higher during the rising phase of an endogenous corticosterone pulse, and these rats are also more aggressive than rats exposed to the intruder during the falling phase of the pulse (Haller et al., 2000a,b).

Glucocorticoid rhythms and the response to stress in physiological and pathological conditions In the previous section, it was discussed how both experimental and mathematical studies provided evidence suggesting that corticosterone ultradian rhythm is important for

Glucocorticoid rhythms and the response to stress in physiological and pathological conditions

111

maintaining optimal hormonal responsiveness to stress (Rankin et al., 2012; Windle et al., 1998b); however, the relationship between the phase of the corticosterone pulse and the response to stress is not always respected. In this section, we will review some experimental models in which circadian and ultradian corticosterone rhythms are different from the “normal” male adult rat, and we will describe how the stress response changes in each of these conditions. Gender. A marked difference in the levels of basal corticosterone secretion between male and female rodents is well recognized (Atkinson and Waddell, 1997), with higher levels in female rats characterized by both an increase in the number of pulses and an increase in pulse magnitude (Seale et al., 2004a). In the rat, gonadal steroids have major effects on the pulsatile pattern of corticosterone. Indeed, following gonadectomy, male rats have increased overall corticosterone secretion similar to that observed in female rats, whereas ovariectomized females have reduced corticosterone with levels similar to those observed in male rats (Seale et al., 2004a). Androgen replacement reverses the increase in corticosterone secretion induced by castration in male rats, and estradiol replacement, in turn, reverses the effects of ovariectomy on corticosterone secretory activity in female rats (Seale et al., 2004b). The differences in the ultradian and circadian corticosterone rhythm observed between male and female rats are also associated with a differential response to both psychological noise stress and LPS-induced immune challenge. These are higher in females than intact male, whereas corticosterone response to stress is similar between female and castrated males but lower in ovariectomized females (Seale et al., 2004a). As seen for basal corticosterone pulsatility, the effects in the stress response are induced by gonadectomy and reversed by androgen and estradiol replacement in males and females, respectively (Seale et al., 2004b). Neonatal masculinization or feminization also affects basal and stress-induced corticosterone secretion patterns in adult life in female and male rats, respectively (Seale et al., 2005a,b). For example, neonatal masculinization results in reduced corticosterone pulsatility in adult female rats, and this effect is associated with reduced corticosterone responses to both noise and LPS stress, as observed in normal adult males (Seale et al., 2005a). In contrast, neonatal deprivation of testosterone in male rats results in increased corticosterone pulsatility and is associated with increased corticosterone response to noise stress or LPS administration (Seale et al., 2005a). Aging. Marked differences in the pattern of corticosterone pulsatility occur across the rat life cycle (Lightman et al., 2000). For example, although corticosterone secretion in juvenile and adults rats is characterized by changes in pulse amplitude over the 24-h cycle resulting in the well-defined circadian rhythm of corticosterone, in elderly (>12 month old) rats, circadian rhythmicity is lost, as a result of decreased pulse amplitude during the peak phase. With respect to the stress, one study reports that aging is associated with increased variability in corticosterone response to stress (Segar et al., 2009); however, another study showed a decreased corticosterone response to restraint in aging rats (Buechel et al., 2014). Reproductive cycle. Over the course of their reproductive cycle, female rats show differences in corticosterone rhythms (Windle et al., 2013). During lactation the circadian rhythm of corticosterone is maintained; however, compared with virgin rats, this is associated with a flattening of the rhythm that is due to a decrease in the evening peak levels, and an increase in the number of corticosterone pulses throughout the 24 h. However, 2 days after experimental weaning, corticosterone levels are significantly suppressed throughout the 24-h

112

8. Rhythms of stress resilience

period, and these effects are reversible, as no differences in either circadian or ultradian rhythms of corticosterone are observed between 13 days postlactating dams and virgin rats. With regard to the stress response during the reproductive cycle, although, as expected, in virgin rats, noise stress causes a rapid increase in plasma corticosterone concentration, in the lactating group, which has an increased number of pulses across the 24-h cycle, no effects on plasma corticosterone levels are seen following noise stress. Furthermore, in the experimentally weaned group, which show lower basal corticosterone levels and a lower number of pulses, the response to noise is of similar amplitude but more prolonged compared with the virgin group (Windle et al., 2013). Genetic background. Differences in ultradian corticosterone rhythms have been investigated across rats with different genetic backgrounds. Studies have shown that Wistar, Sprague Dawley, Lewis, and Fisher 344 all exhibit a pulsatile pattern of corticosterone release. However, significant differences between strains have been observed (Windle et al., 1998a). Similar to male and female Sprague Dawley and Wistar rats, female Lewis rats have a clear circadian rhythm, with greater pulse amplitude in the evening than in the early morning. In contrast, circulating corticosterone concentration is higher in female Fischer rats and lacks circadian variation, with no difference in pulse amplitude between the morning and the evening. In addition to the observed differences in the pattern of ultradian rhythmicity, Fisher and Lewis rats also differ in their corticosterone response to stressors. It is well known that Lewis rats are susceptible to a range of inflammatory conditions, including streptococcal cell walleinduced arthritis (Sternberg et al., 1989a,b), whereas the Fisher rat does not develop these conditions. Because of the antiinflammatory effect of endogenous GCs, these differences in disease susceptibility may be linked to differences in the dynamics of corticosterone secretion. In contrast to inflammatory stress, Fisher rats exhibit a response to noise stress that is greater and more prolonged than in the Lewis female rat. Furthermore, although Fisher rats respond equally to a noise stress, regardless of when it occurs in relation to the phase of the endogenous corticosterone rhythm, Lewis rats lack a corticosterone response when the noise is applied during the falling phase of the ultradian pulse (Windle et al., 1998a). Chronic inflammatory stress. Chronic inflammation is a model of chronic stress that has high clinical relevance. In particular, a model of chronic inflammatory disease that has been extensively studied in the rat is Mycobacterium adjuvanteinduced arthritis. Increased circulating corticosterone and ACTH concentrations, and loss of the normal circadian rhythm of HPA activity, have been observed in Piebald-Viral-Glaxo (PVG) rats infected with the Mycobacterium adjuvant (Windle et al., 2001). During the symptomatic period, infected rats show dramatic changes in pulsatility, with an almost doubled number of pulses throughout the 24-h cycle related to continued pulsatility during the normally quiescent lights-on period. Interestingly, neither the amplitude nor the duration of the pulses is different from control rats, indicating that the increase in circulating hormone is solely due to this increased number of pulses. A different response to stress has been observed in rats exposed to chronic inflammation (adjuvant-induced rheumatoid arthritis). In these rats, the relationship between the pulse phase and the timing of the stress is maintained throughout the development of the inflammation (Windle et al., 2001). However, the overall corticosterone response to noise stress is significantly lower in arthritic rats than in controls. Thus, it is possible that as the number of pulses increases and the interpulse periods reduce in symptomatic rats, there is a greater

Cortisol rhythms and stress resilience in humans

113

proportion of time when the rats are unable to respond to stress. These data provide evidence for a direct relationship between increased basal HPA axis activity, associated with chronic disease, and a decreased response to acute stress, and this is indeed consistent with previous data showing a decreased corticosterone response to acute stress in rats with adjuvant-induced arthritis (Aguilera et al., 1997; Harbuz et al., 1993) and also in humans with rheumatoid arthritis (Chikanza et al., 1992). Interestingly, there is no difference in the corticosterone response to LPS between rats with adjuvant-induced arthritis and control rats. Exposure to constant light. Another model of chronic stress that has been found to effect corticosterone pulsatility in the rat is exposure to constant light. Experimental animals are normally kept under a 12-h light-dark cycle, with light input acting as a zeitgeber processed through the SCN. Disruption of the SCN signal, either by physical lesioning of the SCN or by disruption of the light input, induces a loss of circadian rhythmicity, and this is associated with changes in corticosterone pulsatility (Waite et al., 2012). Rats maintained under conditions of constant light for a prolonged period of time (5 weeks) lose their circadian corticosterone rhythm, which is due to increased pulsatility during the nadir phase of the circadian cycle, with no difference between the nadir and the peak phase pulse amplitude. These same rats have increased levels of CRH mRNA in the morning, which probably accounts for the increased corticosterone secretion during the nadir phase. This suggests that removal of the inhibitory SCN input to the hypothalamic PVN results in sufficient CRH secretion throughout the 24 h to maintain ultradian pituitary-adrenal activity. Consistent with the disinhibitory effect on basal corticosterone, rats exposed to constant light also exhibit a higher corticosterone response to restraint stress. Neonatal programming. Exposure to stress during the neonatal period has profound effects on physiological functions that can be observed in adult life. This phenomenon is known as neonatal programming. Early-life stress can also program the development of the HPA axis with changes that persist for the rest of the life of the organism. The effect of neonatal programming on corticosterone pulsatility in the rat has been investigated using a model of early-life infection (Shanks et al., 2000). In rats exposed to endotoxin (Salmonella enteritidis) in neonatal age (days 3 and 5 postpartum), basal levels of corticosterone are higher both during the nadir and peak phase of the circadian cycle in adult life. This is due to an increase in both the number and amplitude of corticosterone pulses throughout the 24-h cycle. Changes in basal corticosterone pulsatility in rats neonatally exposed to an inflammatory stress are also associated with a differential corticosterone response to stress (Shanks et al., 2000). Rats that receive endotoxin in early life show an increase in both the amplitude and number of corticosterone pulses during basal conditions in adult life and are also hyper responsive to noise and inflammatory stress.

Cortisol rhythms and stress resilience in humans The studies we have described reflect GC circadian and ultradian rhythms and their relevance for stress resilience in rodents, with an emphasis on the effects of physiological (e.g., estrus cycle and aging) and pathological (e.g., chronic stress and inflammation) conditions. These data, however, also have strong clinical implications. Indeed, cortisol secretion in humans is also characterized by both circadian and ultradian rhythmicity, and as observed

114

8. Rhythms of stress resilience

in a number of animal studies, changes in these rhythms can occur in human in pathological states, including depression and other stress-related disorders (Oster et al., 2017). Although responsiveness to stress in humans is highly subjective and social contextedependent (Cohen et al., 2007), a number of human studies have investigated whether there is a daily variation in the cortisol response to acute stress exposure. In these studies, experimental or pharmacological stressors, such as CRH administration, physical exercise, and psychosocial stressors, were used. Cortisol response to CRH is higher in the afternoon or evening, an observation that is consistent with a circadian responsiveness of the adrenal gland to ACTH (Ulrich-Lai et al., 2006). However, as observed in animal studies, circadian responsiveness to stress is also dependent on the type and intensity of the stressor. For example, cortisol secretion following exposure to low to moderate intensity physical exercise was higher in the morning, whereas it was higher in the evening in subject performing high-intensity exercise (Scheen et al., 1998). In contrast, clinical studies have shown no differences between morning and evening in cortisol responses to the Trier Social Stress Test (Kudielka et al., 2004). GCs regulate synaptic plasticity and neurotransmitter activity and are able to modify emotional and cognitive behavior. The maintenance of physiological GC rhythms is therefore extremely important, especially under acute stressful conditions or during poststress cognitive adaptations (Kalafatakis et al., 2016b). To clarify the clinical importance of ultradian rhythmicity in man, ongoing studies are using a model of steroid replacement in which GCs are infused in either a physiological circadian rhythm with its underlying ultradian rhythm or as a constant infusion, which abolishes the ultradian component of the circadian rhythm (Kalafatakis et al., 2016a). By using these methods, researchers are currently testing the hypothesis that the pattern of systemic GC oscillations leads to differential neurobehavioral phenotypes and activation of brain areas related to mood and emotional processing. Specifically, this study will shed light on the impact of GC rhythms on neural processing, emotional reactivity and perception, mood, and self-perceived well-being.

References Aguilera, G., Jessop, D.S., Harbuz, M.S., Kiss, A., Lightman, S.L., 1997. Differential regulation of hypothalamic pituitary corticotropin releasing hormone receptors during development of adjuvant-induced arthritis in the rat. The Journal of Endocrinology 153, 185e191. Atkinson, H.C., Waddell, B.J., 1997. Circadian variation in basal plasma corticosterone and adrenocorticotropin in the rat: sexual dimorphism and changes across the estrous cycle. Endocrinology 138, 3842e3848. Atkinson, H.C., Wood, S.A., Kershaw, Y.M., Bate, E., Lightman, S.L., 2006. Diurnal variation in the responsiveness of the hypothalamic-pituitary-adrenal axis of the male rat to noise stress. Journal of Neuroendocrinology 18, 526e533. Barclay, J.L., Shostak, A., Leliavski, A., Tsang, A.H., Johren, O., Muller-Fielitz, H., Landgraf, D., Naujokat, N., van der Horst, G.T., Oster, H., 2013. High-fat diet-induced hyperinsulinemia and tissue-specific insulin resistance in Crydeficient mice. American Journal of Physiology. Endocrinology and Metabolism 304, E1053eE1063. Bartlang, M.S., Neumann, I.D., Slattery, D.A., Uschold-Schmidt, N., Kraus, D., Helfrich-Forster, C., Reber, S.O., 2012. Time matters: pathological effects of repeated psychosocial stress during the active, but not inactive, phase of male mice. The Journal of Endocrinology 215, 425e437. Bernatova, I., Key, M.P., Lucot, J.B., Morris, M., 2002. Circadian differences in stress-induced pressor reactivity in mice. Hypertension 40, 768e773.

References

115

Bradbury, M.J., Cascio, C.S., Scribner, K.A., Dallman, M.F., 1991. Stress-induced adrenocorticotropin secretion: diurnal responses and decreases during stress in the evening are not dependent on corticosterone. Endocrinology 128, 680e688. Buechel, H.M., Popovic, J., Staggs, K., Anderson, K.L., Thibault, O., Blalock, E.M., 2014. Aged rats are hyporesponsive to acute restraint: implications for psychosocial stress in aging. Frontiers in Aging Neuroscience 6, 13. Buijs, R.M., Wortel, J., Van Heerikhuize, J.J., Kalsbeek, A., 1997. Novel environment induced inhibition of corticosterone secretion: physiological evidence for a suprachiasmatic nucleus mediated neuronal hypothalamo-adrenal cortex pathway. Brain Research 758, 229e236. Cherrington, A.D., 1999. Banting Lecture 1997. Control of glucose uptake and release by the liver in vivo. Diabetes 48, 1198e1214. Chikanza, I.C., Chrousos, G., Panayi, G.S., 1992. Abnormal neuroendocrine immune communications in patients with rheumatoid arthritis. European Journal of Clinical Investigation 22, 635e637. Chrousos, G.P., 1995. The hypothalamic-pituitary-adrenal axis and immune-mediated inflammation. New England Journal of Medicine 332, 1351e1362. Cohen, S., Janicki-Deverts, D., Miller, G.E., 2007. Psychological stress and disease. Journal of the American Medical Association 298, 1685e1687. de Kloet, E.R., 2000. Stress in the brain. European Journal of Pharmacology 405, 187e198. Dunn, J., Scheving, L., Millet, P., 1972. Circadian variation in stress-evoked increases in plasma corticosterone. American Journal of Physiology 223, 402e406. Engeland, W.C., Shinsako, J., Winget, C.M., Vernikos-Danellis, J., Dallman, M.F., 1977. Circadian patterns of stressinduced ACTH secretion are modified by corticosterone responses. Endocrinology 100, 138e147. Fulkerson, W.J., 1978. Synchronous episodic release of cortisol in the sheep. The Journal of Endocrinology 79, 131e132. Gallant, S., Brownie, A.C., 1979. Serum corticosteroids at the high and low points of the circadian rhythm in rats with regenerating adrenals. Life Sciences 24, 1097e1101. Gattermann, R., Weinandy, R., 1996. Time of day and stress response to different stressors in experimental animals. Part I: golden hamster (Mesocricetus auratus Waterhouse, 1839). Journal of Experimental Animal Science 38, 66e76. Gibbs, F.P., 1970. Circadian variation of ether-induced corticosterone secretion in the rat. American Journal of Physiology 219, 288e292. Haller, J., Halasz, J., Mikics, E., Kruk, M.R., Makara, G.B., 2000a. Ultradian corticosterone rhythm and the propensity to behave aggressively in male rats. Journal of Neuroendocrinology 12, 937e940. Haller, J., Millar, S., van de Schraaf, J., de Kloet, R.E., Kruk, M.R., 2000b. The active phase-related increase in corticosterone and aggression are linked. Journal of Neuroendocrinology 12, 431e436. Harbuz, M.S., Rees, R.G., Lightman, S.L., 1993. HPA axis responses to acute stress and adrenalectomy during adjuvant-induced arthritis in the rat. American Journal of Physiology 264, R179eR185. Henley, D.E., Leendertz, J.A., Russell, G.M., Wood, S.A., Taheri, S., Woltersdorf, W.W., Lightman, S.L., 2009. Development of an automated blood sampling system for use in humans. Journal of Medical Engineering and Technology 33, 199e208. Holaday, J.W., Martinez, H.M., Natelson, B.H., 1977. Synchronized ultradian cortisol rhythms in monkeys: persistence during corticotropin infusion. Science 198, 56e58. Jasper, M.S., Engeland, W.C., 1991. Synchronous ultradian rhythms in adrenocortical secretion detected by microdialysis in awake rats. American Journal of Physiology 261, R1257eR1268. Kalafatakis, K., Russell, G.M., Harmer, C.J., Munafo, M.R., Marchant, N., Wilson, A., Brooks, J.C., Thai, N.J., Ferguson, S.G., Stevenson, K., Durant, C., Schmidt, K., Lightman, S.L., 2016a. Effects of the pattern of glucocorticoid replacement on neural processing, emotional reactivity and well-being in healthy male individuals: study protocol for a randomised controlled trial. Trials 17, 44. Kalafatakis, K., Russell, G.M., Zarros, A., Lightman, S.L., 2016b. Temporal control of glucocorticoid neurodynamics and its relevance for brain homeostasis, neuropathology and glucocorticoid-based therapeutics. Neuroscience and Biobehavioral Reviews 61, 12e25. Kalsbeek, A., Ruiter, M., La Fleur, S.E., Van Heijningen, C., Buijs, R.M., 2003. The diurnal modulation of hormonal responses in the rat varies with different stimuli. Journal of Neuroendocrinology 15, 1144e1155.

116

8. Rhythms of stress resilience

Kant, G.J., Mougey, E.H., Meyerhoff, J.L., 1986. Diurnal variation in neuroendocrine response to stress in rats: plasma ACTH, beta-endorphin, beta-LPH, corticosterone, prolactin and pituitary cyclic AMP responses. Neuroendocrinology 43, 383e390. Kudielka, B.M., Schommer, N.C., Hellhammer, D.H., Kirschbaum, C., 2004. Acute HPA axis responses, heart rate, and mood changes to psychosocial stress (TSST) in humans at different times of day. Psychoneuroendocrinology 29, 983e992. Lamia, K.A., Papp, S.J., Yu, R.T., Barish, G.D., Uhlenhaut, N.H., Jonker, J.W., Downes, M., Evans, R.M., 2011. Cryptochromes mediate rhythmic repression of the glucocorticoid receptor. Nature 480, 552e556. Leliavski, A., Shostak, A., Husse, J., Oster, H., 2014. Impaired glucocorticoid production and response to stress in arntl-deficient male mice. Endocrinology 155, 133e142. Lewis, J.G., Bagley, C.J., Elder, P.A., Bachmann, A.W., Torpy, D.J., 2005. Plasma free cortisol fraction reflects levels of functioning corticosteroid-binding globulin. Clinica Chimica Acta 359, 189e194. Lightman, S.L., Windle, R.J., Julian, M.D., Harbuz, M.S., Shanks, N., Wood, S.A., Kershaw, Y.M., Ingram, C.D., 2000. Significance of pulsatility in the HPA axis. Novartis Foundation Symposium 227, 244e257. Discussion 257-260. Lilly, M.P., Jones, R.O., Putney, D.J., Carlson, D.E., 2000. Post-surgical recovery and time-of-day mask potentiated responses of ACTH to repeated moderate hemorrhage in conscious rats. The Journal of Endocrinology 167, 205e217. Lowrey, P.L., Takahashi, J.S., 2011. Genetics of circadian rhythms in Mammalian model organisms. Advances in Genetics 74, 175e230. Macfarlane, D.P., Forbes, S., Walker, B.R., 2008. Glucocorticoids and fatty acid metabolism in humans: fuelling fat redistribution in the metabolic syndrome. The Journal of Endocrinology 197, 189e204. Mathias, S., Schiffelholz, T., Linthorst, A.C., Pollmacher, T., Lancel, M., 2000. Diurnal variations in lipopolysaccharide-induced sleep, sickness behavior and changes in corticosterone levels in the rat. Neuroendocrinology 71, 375e385. McEwen, B.S., 2007. Physiology and neurobiology of stress and adaptation: central role of the brain. Physiological Reviews 87, 873e904. Munck, A., Guyre, P.M., Holbrook, N.J., 1984. Physiological functions of glucocorticoids in stress and their relation to pharmacological actions. Endocrine Reviews 5, 25e44. Oster, H., Challet, E., Ott, V., Arvat, E., de Kloet, E.R., Dijk, D.J., Lightman, S., Vgontzas, A., Van Cauter, E., 2017. The functional and clinical significance of the 24-hour rhythm of circulating glucocorticoids. Endocrine Reviews 38, 3e45. Oster, H., Damerow, S., Kiessling, S., Jakubcakova, V., Abraham, D., Tian, J., Hoffmann, M.W., Eichele, G., 2006. The circadian rhythm of glucocorticoids is regulated by a gating mechanism residing in the adrenal cortical clock. Cell Metabolism 4, 163e173. Panda, S., Hogenesch, J.B., Kay, S.A., 2002. Circadian rhythms from flies to human. Nature 417, 329e335. Rankin, J., Walker, J.J., Windle, R., Lightman, S.L., Terry, J.R., 2012. Characterizing dynamic interactions between ultradian glucocorticoid rhythmicity and acute stress using the phase response curve. PLoS One 7, e30978. Retana-Marquez, S., Bonilla-Jaime, H., Vazquez-Palacios, G., Dominguez-Salazar, E., Martinez-Garcia, R., VelazquezMoctezuma, J., 2003. Body weight gain and diurnal differences of corticosterone changes in response to acute and chronic stress in rats. Psychoneuroendocrinology 28, 207e227. Sarabdjitsingh, R.A., Conway-Campbell, B.L., Leggett, J.D., Waite, E.J., Meijer, O.C., de Kloet, E.R., Lightman, S.L., 2010. Stress responsiveness varies over the ultradian glucocorticoid cycle in a brain-region-specific manner. Endocrinology 151, 5369e5379. Scheen, A.J., Buxton, O.M., Jison, M., Van Reeth, O., Leproult, R., L’Hermite-Baleriaux, M., Van Cauter, E., 1998. Effects of exercise on neuroendocrine secretions and glucose regulation at different times of day. American Journal of Physiology 274, E1040eE1049. Seale, J.V., Wood, S.A., Atkinson, H.C., Bate, E., Lightman, S.L., Ingram, C.D., Jessop, D.S., Harbuz, M.S., 2004a. Gonadectomy reverses the sexually diergic patterns of circadian and stress-induced hypothalamic-pituitary-adrenal axis activity in male and female rats. Journal of Neuroendocrinology 16, 516e524. Seale, J.V., Wood, S.A., Atkinson, H.C., Harbuz, M.S., Lightman, S.L., 2004b. Gonadal steroid replacement reverses gonadectomy-induced changes in the corticosterone pulse profile and stress-induced hypothalamic-pituitaryadrenal axis activity of male and female rats. Journal of Neuroendocrinology 16, 989e998.

References

117

Seale, J.V., Wood, S.A., Atkinson, H.C., Harbuz, M.S., Lightman, S.L., 2005a. Postnatal masculinization alters the HPA axis phenotype in the adult female rat. The Journal of Physiology 563, 265e274. Seale, J.V., Wood, S.A., Atkinson, H.C., Lightman, S.L., Harbuz, M.S., 2005b. Organizational role for testosterone and estrogen on adult hypothalamic-pituitary-adrenal axis activity in the male rat. Endocrinology 146, 1973e1982. Segar, T.M., Kasckow, J.W., Welge, J.A., Herman, J.P., 2009. Heterogeneity of neuroendocrine stress responses in aging rat strains. Physiology and Behavior 96, 6e11. Shanks, N., Windle, R.J., Perks, P.A., Harbuz, M.S., Jessop, D.S., Ingram, C.D., Lightman, S.L., 2000. Early-life exposure to endotoxin alters hypothalamic-pituitary-adrenal function and predisposition to inflammation. Proceedings of the National Academy of Sciences of the United States of America 97, 5645e5650. Spiga, F., Walker, J.J., Terry, J.R., Lightman, S.L., 2014. HPA axis-rhythms. Comparative Physiology 4, 1273e1298. Sternberg, E.M., Hill, J.M., Chrousos, G.P., Kamilaris, T., Listwak, S.J., Gold, P.W., Wilder, R.L., 1989a. Inflammatory mediator-induced hypothalamic-pituitary-adrenal axis activation is defective in streptococcal cell wall arthritissusceptible Lewis rats. Proceedings of the National Academy of Sciences of the United States of America 86, 2374e2378. Sternberg, E.M., Young 3rd, W.S., Bernardini, R., Calogero, A.E., Chrousos, G.P., Gold, P.W., Wilder, R.L., 1989b. A central nervous system defect in biosynthesis of corticotropin-releasing hormone is associated with susceptibility to streptococcal cell wall-induced arthritis in Lewis rats. Proceedings of the National Academy of Sciences of the United States of America 86, 4771e4775. Tapp, W.N., Holaday, J.W., Natelson, B.H., 1984. Ultradian glucocorticoid rhythms in monkeys and rats continue during stress. American Journal of Physiology 247, R866eR871. Torrellas, A., Guaza, C., Borrell, J., Borrell, S., 1981. Adrenal hormones and brain catecholamines responses to morning and afternoon immobilization stress in rats. Physiology and Behavior 26, 129e133. Turek, F.W., Joshu, C., Kohsaka, A., Lin, E., Ivanova, G., McDearmon, E., Laposky, A., Losee-Olson, S., Easton, A., Jensen, D.R., Eckel, R.H., Takahashi, J.S., Bass, J., 2005. Obesity and metabolic syndrome in circadian clock mutant mice. Science 308, 1043e1045. Ulrich-Lai, Y.M., Arnhold, M.M., Engeland, W.C., 2006. Adrenal splanchnic innervation contributes to the diurnal rhythm of plasma corticosterone in rats by modulating adrenal sensitivity to ACTH. American Journal of Physiology - Regulatory, Integrative and Comparative Physiology 290, R1128eR1135. Waite, E.J., McKenna, M., Kershaw, Y., Walker, J.J., Cho, K., Piggins, H.D., Lightman, S.L., 2012. Ultradian corticosterone secretion is maintained in the absence of circadian cues. European Journal of Neuroscience 36, 3142e3150. Weitzman, E.D., Fukushima, D., Nogeire, C., Roffwarg, H., Gallagher, T.F., Hellman, L., 1971. Twenty-four hour pattern of the episodic secretion of cortisol in normal subjects. The Journal of Cinical Endocrinology and Metabolism 33, 14e22. Windle, R.J., Wood, S.A., Kershaw, Y.M., Lightman, S.L., Ingram, C.D., 2013. Adaptive changes in basal and stressinduced HPA activity in lactating and post-lactating female rats. Endocrinology 154, 749e761. Windle, R.J., Wood, S.A., Kershaw, Y.M., Lightman, S.L., Ingram, C.D., Harbuz, M.S., 2001. Increased corticosterone pulse frequency during adjuvant-induced arthritis and its relationship to alterations in stress responsiveness. Journal of Neuroendocrinology 13, 905e911. Windle, R.J., Wood, S.A., Lightman, S.L., Ingram, C.D., 1998a. The pulsatile characteristics of hypothalamo-pituitaryadrenal activity in female Lewis and Fischer 344 rats and its relationship to differential stress responses. Endocrinology 139, 4044e4052. Windle, R.J., Wood, S.A., Shanks, N., Lightman, S.L., Ingram, C.D., 1998b. Ultradian rhythm of basal corticosterone release in the female rat: dynamic interaction with the response to acute stress. Endocrinology 139, 443e450. Yang, S., Liu, A., Weidenhammer, A., Cooksey, R.C., McClain, D., Kim, M.K., Aguilera, G., Abel, E.D., Chung, J.H., 2009. The role of mPer2 clock gene in glucocorticoid and feeding rhythms. Endocrinology 150, 2153e2160. Zhang, J., Wu, Z., Zhou, L., Li, H., Teng, H., Dai, W., Wang, Y., Sun, Z.S., 2011. Deficiency of antinociception and excessive grooming induced by acute immobilization stress in Per1 mutant mice. PLoS One 6, e16212. Zimmermann, E., Critchlow, V., 1967. Effects of diurnal variation in plasma corticosterone levels on adrenocortical response to stress. Proceedings of the Society for Experimental Biology and Medicine 125, 658e663.

C H A P T E R

9

Mitochondrial function and stress resilience Laia Morató, Carmen Sandi Laboratory of Behavioral Genetics, Brain Mind Institute, École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland

Introduction Stress is a major risk factor for the development of psychopathologies, such as depression or posttraumatic stress disorder (PTSD). However, extensive data from both animal and human studies highlight the existence of major differences in individuals’ susceptibility to stress (Duclot and Kabbaj, 2013; McEwen et al., 2015; Russo et al., 2012). This has been particularly well documented for depression, with broad evidence showing that while some individuals show high vulnerability to develop depressive symptoms following stress exposure, others remain resilient (Russo et al., 2012; Sandi and Richter-Levin, 2009). Similarly, PTSD studies are starting to place increasing emphasis on identifying factors that explain individual differences in response to traumatic stress exposure and promotion of resilience (Yehuda et al., 2015). The identification of mechanisms underlying stress-resilient phenotypes has proved difficult most probably due to the complexity of the gene-by-environment (G
E) interactions inherent to stress-related disorders (Sharma et al., 2016). Recently, genetic studies have started to identify a set of genes whose differential expression can affect stress responsiveness (Klok et al., 2011; Wong et al., 2017) and interindividual differences in resilience to stress-induced psychopathology (Klok et al., 2011; Nievergelt et al., 2015; Wong et al., 2017). Importantly, some of the identified genetic variants correspond to genes that regulate different aspects of mitochondrial function and energy metabolism (Cai et al., 2015a; Czarny et al., 2018; Flaquer et al., 2015; Kishi et al., 2010; Kovanen et al., 2015; Libert et al., 2011). These genetic insights, along with cumulative evidence underscoring alterations in mitochondrial function following stress exposure and in the context of stress-related psychopathologies (Gong et al., 2011; Manji et al., 2012; Morava and Kozicz, 2013; Picard et al., 2014, 2015), suggest that optimal mitochondrial functioning could be at the core of stress

Stress Resilience https://doi.org/10.1016/B978-0-12-813983-7.00009-4

119

Copyright © 2020 Elsevier Inc. All rights reserved.

120

9. Mitochondrial function and stress resilience

resilience. This view has been principally supported by recent rodent studies highlighting differences in the functioning of mitochondria in the nucleus accumbensda brain region involved in stress adaptationdin association with individual differences in trait anxiety (Hollis et al., 2015; Larrieu et al., 2017; van der Kooij et al., 2018). This link is especially relevant because high trait anxiety is a phenotype particularly vulnerable to stress and stress-related psychopathologies. In this chapter, we will present key findings in this emerging research field to illustrate how various aspects related to mitochondrial function can contribute to differential stress responses and susceptibility. We will start by briefly describing the essentials of mitochondria structure and function and their contributions to synaptic processes and interactions with glucocorticoids. Subsequently, we will summarize evidence from human and animal studies that support the hypothesis that mitochondrial function and stress resilience are closely connected. Finally, we will discuss the therapeutic potential of boosting mitochondrial function to combat stress-related disorders.

The mitochondrion Mitochondria are complex and highly dynamic organelles that divide, fuse, and move throughout the cell. They have double membrane (i.e., inner and outer mitochondrial membrane) and their own genomic material, known as mitochondrial DNA (mtDNA). Mitochondria are considered the powerhouses of the cell because of their efficient capacity to produce energy in form of adenosine triphosphate (ATP) through the oxidative phosphorylation (OXPHOS) system. In a nutshell, the oxidation of nutritional substrates through a variety of catabolic reactions (i.e., glycolysis, fatty acid beta-oxidation or tricarboxylic acid [TCA] cycle) gives rise to reducing equivalents, named nicotinamide adenine dinucleotide (NADH) and flavoadenine dinucleotide (FADH2). Electrons from these reducing equivalents are transferred through the complexes I to IV of the electron transfer chain (ETC) generating a proton gradient across the inner mitochondrial membrane. Subsequently, ATP synthase (or complex V) uses the energy stored in this electrochemical gradient to convert ADP into ATP (Reeve et al., 2011). In addition to their bioenergetic function, mitochondria play a very prominent role in several other processes, including calcium (Ca2þ) homeostasis and production of reactive oxygen species (ROS). Through the expression of the mitochondrial calcium uniporter, located in the inner mitochondrial membrane, mitochondria can rapidly uptake free Ca2þ from the cytoplasm arising from internal stores or extracellular influx (Kamer and Mootha, 2015). Conversely, mitochondria can release Ca2þ to the cytosol through a Naþ/Ca2þ exchanger expressed in the inner mitochondrial membrane (Palty et al., 2012). On the other hand, mitochondria ROS, which are partially reduced intermediates of oxygen (e.g., superoxide   produce anion O2  , hydrogen peroxide [H2O2], or hydroxyl radical [OH]), generated as byproducts of the electron transfer at the complexes I and III of the ETC. Oxidative stress and its damaging effects on proteins, lipids, and DNA occur when the equilibrium between ROS production and antioxidant capacity is disrupted. Neurons are especially vulnerable to oxidative damage because of their high rate of oxidative metabolic activity, their low antioxidant capacity, and the high abundance of peroxidizable polyunsaturated fatty acids in

Mitochondria and glucocorticoids

121

neuronal membranes in addition to their nonreplicative nature (Murphy, 2009). However, it is important to note that ROS are not only deleterious to cell function, but they also serve important regulatory functions notably acting as signaling molecules capable of modulating gene transcription and epigenetic changes (Schieber and Chandel, 2014). Finally, mitochondria are also required for other biological processes, which will be less discussed in this chapter, such as the activation of apoptotic cell death and the biosynthesis of several macromolecules, including steroid hormones (Reeve et al., 2011).

Mitochondria in neurotransmission and synaptic plasticity The brain has high energy requirements. Despite representing only 2% of the total body mass, it makes use of around 20% of the total oxygen and 25% of the total glucose consumed by the organism (Mink et al., 1981). To meet activity-related high metabolic demands, mitochondria are recruited to activated synapses where ATP is essential for successful neurotransmission and neuroplasticity (Harris et al., 2012). Specifically, mitochondrial functions contribute to basic processes in both the pre- and the postsynaptic compartments. Presynaptically, ATP is fundamental for the transport of synaptic vesicles, the release and recycle of neurotransmitters, and the regulation of ATP-powered ionic pumps critically implicated in the propagation of action potentials (Lord et al., 2013; Marland et al., 2016; Pathak et al., 2015; Rangaraju et al., 2014; Sun et al., 2013; Verstreken et al., 2005). In addition, TCA cycle intermediates generated in the mitochondria serve as the building blocks for the synthesis of GABA and glutamate (Sibson et al., 1998; Waagepetersen et al., 2001). Postsynaptically, both ATP production and mitochondrial Ca2þ buffering are required for the reversal of ionic gradients produced following neuronal excitation (Harris et al., 2012). Importantly, synaptic mitochondria are more susceptible to Ca2þ overload and alteration in the ETC compared with nonsynaptic mitochondria (Yarana et al., 2012). Mitochondrial Ca2þ buffering has also been implicated in the production of long-term potentiation (Billups and Forsythe, 2002; Delaney and Tank, 1994; Kang et al., 2008; Levy et al., 2003; Stanton and Schanne, 1986; Tang and Zucker, 1997; Yang et al., 2003; Yarana et al., 2012), a learningrelated plasticity phenomenon highly susceptible of modulation by stress (Luksys and Sandi, 2011). Extension or migration of mitochondria into dendritic protrusions correlates with the development and maintenance of dendrites as well as synapse and spine formation (Li et al., 2004), two neurobiological processes extremely sensitive to regulation by experience and stress (McEwen and Chattarji, 2004).

Mitochondria and glucocorticoids Classically, the mode of action of the activated glucocorticoid receptor (GR) involves the translocation from the cytoplasm to the nucleus where it can modulate transcription of a large number of genes. Noteworthy, some of the GR-regulated genes include nuclear-encoded mitochondrial genes. Recent studies have demonstrated that activated, ligand-bound GRs can also translocate to the mitochondria, where they can regulate expression of genes

122

9. Mitochondrial function and stress resilience

encoded in the mitochondrial genome. Indeed, mtDNA contains sequences with strong homology to glucocorticoid response elements (Psarra and Sekeris, 2011). Pioneering studies in cortical cultured neurons showed that physiological doses of glucocorticoids promote mitochondrial function, as indicated by increased mitochondrial membrane potential and Ca2þ buffering capacity along with neuroprotective effects against kainic acideinduced apoptosis. However, supraphysiological doses of glucocorticoids exert the opposite effects in the abovementioned mitochondrial functions and enhance the kainic acid apoptotic effects (Du et al., 2009). These studies further showed that whereas acute treatment with both physiological and supraphysiological doses of glucocorticoids promotes GR translocation into the mitochondria, the extent of GR translocation to the mitochondrial was reduced under long-term glucocorticoid treatments involving supraphysiological doses (Du et al., 2009). This finding was validated in in vivo experiments that showed that chronic treatment with supraphysiological glucocorticoid doses downregulates mitochondrial GR levels in the prefrontal cortex (Du et al., 2009). Recently, chronic treatment with either corticosterone or stress was shown to increase GR binding to mtDNA in the rat hippocampus along with an upregulation of several mtDNAencoded genes (e.g., Nd-3, Nd-4, Cox-2, Nd-4l, Atp-6, Atp-8, Nd-5, Cox-3, Cox-1, and Cytb). On the contrary, acute stress was shown to downregulate the expression of mtDNAencoded complex I subunit genes (Hunter et al., 2016). Collectively, these results indicate that glucocorticoids can increase the capacity of mitochondria to meet the rise in energy demands required for a successful adaptation of the brain to stress. However, sustained glucocorticoid actions on mitochondria might eventually lead to maladaptive effects.

Mitochondrial dysfunction in stress-related disorders: human studies A first line of evidence linking mitochondrial dysfunction with stress-related disorders is provided by clinical observations from patients with mitochondriopathies. Mitochondrial disorders are typically caused by mutations of genes encoded either in the mtDNA or in the nuclear DNA that affect the OXPHOS system. Although mitochondrial diseases are multisystemic disorders, the brain is the most commonly affected organ (DiMauro et al., 2013). Of note in the context of this chapter, patients with mitochondrial disorders present a high incidence of psychopathologies, including anxiety and depression. Moreover, the onset of anxiety and depression symptoms in these patients typically precedes the appearance of primary mitochondrial disorder symptoms (Fattal et al., 2007; Inczedy-Farkas et al., 2012; Koene et al., 2009; Morava et al., 2010; Morava and Kozicz, 2013). Remarkably, brain metabolic dysfunction might contribute to anxiety and mood changes in these patients, as suggested by data from 1H-magnetic resonance spectroscopy (1H-MRS) revealing correlations between several metabolites (e.g., N-acetyl aspartate, creatine, glycerophosphocholine, myoinositol, and glutamate þ glutamine [Glx]) in the hippocampus and anxiety levels (Anglin et al., 2012). In addition, converging data from neuroimaging, proteomic, genomic, and genetic approaches are pointing to alterations in mitochondrial function in stress-related disorders, such as depression and PTSD. For example, several positron emission tomography (PET) studies have highlighted a downregulation of cerebral metabolism in depressed patients, particularly affecting brain regions critically involved in the regulation of mood, such as

Mitochondrial dysfunction in stress-related disorders: human studies

123

the prefrontal cortex, basal ganglia, and anterior cingulate gyrus (Su et al., 2014; Videbech, 2000). Evidence supporting that this hypofunctionality is potentially related to alterations in energy metabolism is provided by a number of magnetic resonance spectroscopy (MRS) studies in depressed patients. Using 31P-MRS, low concentration of ATP was detected in the basal ganglia of depressed subjects (Moore et al., 1997). Importantly, combination of spectroscopy (i.e., 1H-MRS and 13C-MRS) with infusion of [1-13C] glucose revealed that mitochondrial energy production is 26% lower in occipital glutamatergic neurons in these patients (Abdallah et al., 2014). The existence of dysfunctional mitochondria in these patients is further supported by postmortem proteomic and genomic studies focusing on specific brain regions. To illustrate, proteomic studies focusing on the prefrontal cortex and anterior cingulate cortex of patients with depression showed alterations in the expression of proteins coded by mtDNA, predominantly of the OXPHOS system (Beasley et al., 2006; Gottschalk et al., 2014; Johnston-Wilson et al., 2000; Martins-de-Souza et al., 2012; Zuccoli et al., 2017). Notably, these changes in the metabolism of the brain might be region specific. This possibility is supported by a study in which alterations in the expression of three complex I subunits (i.e., NDUFV1, NDUFV2, and NDUFS1) in depressive patients were observed in the cerebellum, but not in the prefrontal cortex, striatum, or parietooccipital cortex (Ben-Shachar and Karry, 2008). Similarly, neuroimaging studies in PTSD patients, including PET and MRS, have also highlighted alterations in cerebral metabolism and perfusion, receptor binding, and metabolite profiles in limbic regions, medial prefrontal cortex, and temporal cortex (Im et al., 2016). The potential link between these findings and mitochondrial dysfunction is supported by a growing body of data. For example, PTSD-specific expression fingerprints of 800 informative mitochondria genes were reported using human mitochondria-focused cDNA microarrays in postmortem samples from the dorsolateral prefrontal cortex (Su et al., 2008). The largest group of dysregulated genes corresponded to genes involved in mitochondrial dysfunction, oxidative phosphorylation, and cell survival and apoptosis (Su et al., 2008). Analysis of peripheral blood cells from depressive and PTSD patients revealed that mitochondrial abnormalities are not restricted to the brain. A reduction of mtDNA copy number in these cells has been reported in patients diagnosed with depression or PTSD (Bersani et al., 2016). In addition, mitochondrial respiration was found to be lower in depressive patients than in controls and correlated negatively with the severity of depressive symptoms (Karabatsiakis et al., 2014). Dysregulation of several ETC subunits was also found in blood samples of PTSD patients, which also exhibited altered glycolysis and TCA metabolism (Zhang et al., 2015). However, it should be noted that some studies have reported opposite evidence to the one indicated above. To illustrate, a study reported increased mtDNA in depressive patients and a correlation between the amount of mtDNA and the total number of stressful life events (Cai et al., 2015b). Clearly, this emerging field is still in development, and further studies are needed to obtain a clear picture of the contribution of different aspects of mitochondrial function to these disorders. Moreover, several studies suggest that oxidative stress and inefficient repair mechanisms of DNA damage may contribute to the development of stress-related psychopathologies. Evidence includes increased levels of reactive oxygen and nitrogen species (Czarny et al., 2018) as well as increased mtDNA oxidative damage in depressed patients. Although direct measurements of oxidative stress in traumatized humans are still missing, its potential relevance

124

9. Mitochondrial function and stress resilience

for PTSD is bolstered by a mounting body of data showing high levels of oxidative damage markers in blood from stressed individuals (Miller and Sadeh, 2014). In agreement with the hypothesis that oxidative stress plays a role in PTSD-associated neurodegeneration, singlenucleotide polymorphisms (SNPs) in oxidative stress-related genes moderate the association between PTSD and reduced thickness of the right prefrontal cortex in PTSD patients (Miller et al., 2015). Similarly, genetic variants in oxidative stress-related genes have also been associated with vulnerability to develop depression (Czarny et al., 2018) and PTSD (Flaquer et al., 2015). Interestingly, evidence for PTSD includes the identification of mitochondrial singlenucleotide polymorphisms (mtSNPs) in the ATP synthase subunit 8 (MT-ATP8) and the NADH dehydrogenase subunit 5 (MT-ND5) genes, both linked to the regulation of mitochondrial ROS (Flaquer et al., 2015). The idea that genetic variation may alter the effectiveness of mitochondrial function to respond to stress and, hence, influence stress resilience, goes well beyond genes related to oxidative stress. Notably, polymorphisms in the SIRT1 gene were first associated with anxiety (Libert et al., 2011) and depression (Kishi et al., 2010). Sirtuin 1 (SIRT1) is a NADþdependent deacetylase that regulates mitochondrial function and biogenesis through the deacetylation and activation of transcriptional regulators such as FOXO and PGC-1a (Houtkooper et al., 2012). Importantly, a recent study showed that a SNP in the locus 50 in the SIRT1 gene exceeds genome-wide significance in association with recurrent major depressive disorder in two different cohorts of Chinese patients. However, comparison with results from the Psychiatric Genomics Consortium (PGC), a megaanalysis of European studies, failed to provide robust replication for this SNP, which might be explained by differences in sample ascertainment or ethnicity (Cai et al., 2015b). Future studies are warranted to clarify the relevance of this gene in the context of anxiety and depression.

Stress effects in mitochondrial function: animal studies Alterations in brain mitochondrial function have been described in animals subjected to stress, frequently consisting of body restraint. Early studies showed that severe acute immobilization stress in rats induces oxidative damage, not only in plasma and liver, but also in lipid, protein, and DNA of several brain areas (Liu et al., 1996). Recently, a milder acute stress version consisting of 30 min of restraint stress in rats was found to inhibit complex I activity in the brain (Batandier et al., 2014). Other studies showed that exposure of rats to a combination of restraint stress and tail shock protocoldwhich induces PTSD-like behaviorsd upregulates TCA cycle genes and a subunit of the ATP synthase (complex V) (Li et al., 2014; Zhang et al., 2015). Given the considerable impact of acute stress in mitochondrial function, it is not surprising that chronic stress leads also to substantial effects in a broad variety of parameters. Specifically, several studies involving chronic mild stress in rats were found to lead to inhibition of complexes I, III, and IV in the cortex and cerebellum (Rezin et al., 2008), as well as increased generation of ROS in the hippocampus and prefrontal cortex (Lucca et al., 2009). They also showed alterations in mitochondrial ultrastructure, inhibition of mitochondrial respiration, and dissipation of the inner mitochondrial membrane potential in hippocampus,

Stress effects in mitochondrial function: animal studies

125

cortex, and hypothalamus (Gong et al., 2011). Using metabolomics and proteomics approaches, a study focusing on the cerebellum confirmed that chronic mild stress alters glycolysis, the TCA cycle, and ATP synthesis (Shao et al., 2015). The studies discussed above clearly show that stress can affect different components of mitochondrial function. However, they do not provide enough information to establish a link between mitochondria and stress resilience. Evidence for such a link has started to be provided by studies involving different living conditions and a focus on social structures. Using a proteomic approach, environmental enrichment was found to affect both basal and acute restraint stress-induced levels of mitochondria-related proteins in the nucleus accumbens, a brain region that mediates neural adaptations involved in certain stress-induced depressive-like behaviors. Specifically, whereas the levels of proteins involved in the TCA cycle and ETC were lower in environmentally enriched than in socially isolated rats under basal conditions, the two groups showed opposite modulation in the expression of these proteins in response to acute stress, that is, increased in enriched rats, whereas decreased in socially isolated rats (Fan et al., 2013). Moreover, a recent 1H-NMR study in mice reported a differential metabolic profile in the nucleus accumbens in relation to both social status and vulnerability to chronic social defeat. Thus, subordinate mice showed lower levels of several energy-related metabolites (i.e., creatine and phosphocreatine, glutamate, glutamine, aspartate, myoinositol, N-acetyl-aspartate, and taurine) than dominant mice under basal conditions. However, the levels of these metabolites following chronic social defeat stress increased only in subordinate mice (Larrieu et al., 2017). Importantly, recent studies using genetically modified mice presenting either deletions or mutations in specific mitochondrial genes have causally implicated mitochondrial function in stress responsiveness. Mice deficient for Nd6 and Co1 (subunits of complex I and complex IV of the ETC, respectively) or Ant (mitochondrial ATP-ADP translocator) presented an altered response to acute restraint stress (Picard et al., 2015). Two mouse studies that focused on SIRT1 reported increased vulnerability to chronic social defeat following viral-mediated overexpression of SIRT1 in the nucleus accumbens (Kim et al., 2016), suggesting a regionspecific contribution of this protein to stress resilience. However, and surprisingly, the same viral-mediated overexpression of SIRT1 in the hippocampus led to opposite results (Abe-Higuchi et al., 2016). Other studies have investigated whether manipulation of mtDNA influences mice behavior, although not yet on stress responsiveness. These studies have shown, for example, that mice harboring a mutant form of Polg (the mtDNA polymerase) exclusively in neurons exhibit depressive-like behaviors concomitant with mtDNA deletions and mitochondrial dysfunction (Kasahara et al., 2016). The presence of more than one mtDNA variantda state known as heteroplasmydcan also impact behavior. Mice carrying a mixture of mtDNA from the NZB and the 129 mouse strains displayed less depressive- and anxiety-related behaviors than control homoplasmic mice (i.e., mice with only one type of mtDNA, either from NZB or the 129 background) (Sharpley et al., 2012). Another line of evidence linking mitochondrial function with vulnerability to stress is provided by studies that take into account individual differences in anxiety trait. Highanxiety trait is a vulnerability factor to develop stress-induced depression (Castro et al., 2012; Sandi et al., 2008; Sandi and Richter-Levin, 2009). Notably, mice selectively bred for

126

9. Mitochondrial function and stress resilience

more than 40 generations for high or low anxiety-like behaviors revealed perturbations in cellular metabolism and antioxidant responses, when studied in the cingulate cortex. Particularly, highly anxious animals showed decreased glycolysis, pentose phosphate pathway, and antioxidant defense together with increased OXPHOS, TCA cycle, and mitochondrial transport (Filiou et al., 2011). Strikingly, oral administration of a mitochondria-targeted antioxidant (MitoQ) was effective to exert anxiolytic effects in the high anxious line (Nussbaumer et al., 2016). Furthermore, decreased mitochondrial respiration, ATP production, and complex I and II expression together with increased ROS production were also observed in the nucleus accumbens of high-anxious outbred rats compared with lowanxious rats. Remarkably, microinfusion of specific mitochondrial complex I or II inhibitors (malonic acid or 3NP) into the nucleus accumbens significantly reduced social rank, mimicking the low probability to become dominant observed in high-anxious animals. Conversely, infusion of the mitochondria booster nicotinamide was able to prevent the subordinate status of high-anxious individuals (Hollis et al., 2015; van der Kooij et al., 2018). Altogether, the studies summarized in this chapter present converging evidence in support of the view that optimal mitochondrial function might be a critical mechanism in stress coping and facilitation of stress resilience. Accordingly, facilitating mitochondrial function might be a plausible way to improve individuals’ capacity to cope with stress and, hence, to promote resilience.

Promoting stress resilience through activation of mitochondrial function Interestingly, classical antidepressants (e.g., desipramine, fluoxetine, or imipramine) have been shown to modulate mitochondrial function (Adzic et al., 2016; Villa et al., 2017) and reduce oxidative stress (Liu et al., 2015). Moreover, antidepressant effects of ketamine have been ascribed to the activation of energy metabolism and the antioxidant defense system (Weckmann et al., 2017). Other work has shown that compounds that promote mitochondrial function and/or redox homeostasis when given either alone or in combination with existing therapies ameliorate symptoms of stress-related disorders. For instance, the use of antioxidants such as N-acetyl cysteine alone or as supplements has provided promising effects in clinical trials for depression (Berk et al., 2008) and PTSD (Back et al., 2016). In the same line, dietary polyphenol or antioxidant intakes were positively associated with psychological resilience in healthy subjects (Bonaccio et al., 2018). A further example includes evidence from clinical trials indicating that the efficacy of the antidepressant medication was accelerated by the supplementation of patients with creatine monohydrate, a metabolite that facilitates the recycling of ATP (Kondo et al., 2011; Lyoo et al., 2012; Nemets and Levine, 2013). Furthermore, mitochondrial function can be pharmacologically boosted with resveratrol, a natural activator of SIRT1. This polyphenol was found to exert antidepressive effects in mice subjected to early-social isolation (Lo Iacono et al., 2015) and in rats subjected to chronic unpredictable mild stress (Ge et al., 2013; Liu et al., 2014). In addition, dietary intake of glucoraphanin, a compound that promotes the expression of antioxidant enzymes, confers stress resilience in mice subjected to social defeat (Yao et al., 2016).

References

127

Conclusions and future perspectives Mitochondria are primarily responsible for meeting the high-energy demand of the brain. This dependence on energy makes neurons particularly vulnerable to mitochondrial dysfunction. Indeed, mitochondrial dysfunction is a hallmark of neurodegenerative diseases (Reeve et al., 2011). Therefore, it is not surprising that optimal mitochondrial function is also essential to cover the energy requirements associated to stress adaptation within the brain. In this chapter, we have compiled results from multiple studies that demonstrate that mitochondrial function is altered in stress-related disorders, such as depression and PTSD, and in mice subjected to a variety of stress protocols. We highlight mitochondria as an important cellular resilience mechanism and hypothesize that enhancing mitochondrial function may represent a novel therapeutic strategy to treat stress-induced disorders. Future studies should try to elucidate which brain regions and neuronal types are more vulnerable to mitochondrial dysfunction upon stress. They should also address whether mitochondrial dysfunction in animals subjected to stress is permanent or reversible.

References Abdallah, C.G., Jiang, L., De Feyter, H.M., Fasula, M., Krystal, J.H., Rothman, D.L., Mason, G.F., Sanacora, G., 2014. Glutamate metabolism in major depressive disorder. American Journal of Psychiatry 171 (12), 1320e1327. Abe-Higuchi, N., Uchida, S., Yamagata, H., Higuchi, F., Hobara, T., Hara, K., Kobayashi, A., Watanabe, Y., 2016. Hippocampal sirtuin 1 signaling mediates depression-like behavior. Biological Psychiatry 80 (11), 815e826. Adzic, M., Brkic, Z., Bulajic, S., Mitic, M., Radojcic, M.B., 2016. Antidepressant action on mitochondrial dysfunction in psychiatric disorders. Drug Development Research 77 (7), 400e406. Anglin, R.E., Rosebush, P.I., Noseworthy, M.D., Tarnopolsky, M., Mazurek, M.F., 2012. Psychiatric symptoms correlate with metabolic indices in the hippocampus and cingulate in patients with mitochondrial disorders. Translational Psychiatry 2, e187. Back, S.E., McCauley, J.L., Korte, K.J., Gros, D.F., Leavitt, V., Gray, K.M., Hamner, M.B., DeSantis, S.M., Malcolm, R., Brady, K.T., Kalivas, P.W., 2016. A double-blind, randomized, controlled pilot trial of N-acetylcysteine in veterans with posttraumatic stress disorder and substance use disorders. Journal of Clinical Psychiatry 77 (11), e1439ee1446. Batandier, C., Poulet, L., Hininger, I., Couturier, K., Fontaine, E., Roussel, A.M., Canini, F., 2014. Acute stress delays brain mitochondrial permeability transition pore opening. Journal of Neurochemistry 131 (3), 314e322. Beasley, C.L., Pennington, K., Behan, A., Wait, R., Dunn, M.J., Cotter, D., 2006. Proteomic analysis of the anterior cingulate cortex in the major psychiatric disorders: evidence for disease-associated changes. Proteomics 6 (11), 3414e3425. Ben-Shachar, D., Karry, R., 2008. Neuroanatomical pattern of mitochondrial complex I pathology varies between schizophrenia, bipolar disorder and major depression. PLoS One 3 (11), e3676. Berk, M., Copolov, D.L., Dean, O., Lu, K., Jeavons, S., Schapkaitz, I., Anderson-Hunt, M., Bush, A.I., 2008. N-acetyl cysteine for depressive symptoms in bipolar disorder–a double-blind randomized placebo-controlled trial. Biological Psychiatry 64 (6), 468e475. Bersani, F.S., Morley, C., Lindqvist, D., Epel, E.S., Picard, M., Yehuda, R., Flory, J., Bierer, L.M., Makotkine, I., AbuAmara, D., Coy, M., Reus, V.I., Lin, J., Blackburn, E.H., Marmar, C., Wolkowitz, O.M., Mellon, S.H., 2016. Mitochondrial DNA copy number is reduced in male combat veterans with PTSD. Progress in NeuroPsychopharmacology and Biological Psychiatry 64, 10e17. Billups, B., Forsythe, I.D., 2002. Presynaptic mitochondrial calcium sequestration influences transmission at mammalian central synapses. Journal of Neuroscience 22 (14), 5840e5847. Bonaccio, M., Di Castelnuovo, A., Costanzo, S., Pounis, G., Persichillo, M., Cerletti, C., Donati, M.B., de Gaetano, G., Iacoviello, L., 2018. Mediterranean-type diet is associated with higher psychological resilience in a general adult population: findings from the Moli-sani study. European Journal of Clinical Nutrition 72 (1), 154e160.

128

9. Mitochondrial function and stress resilience

Cai, N., Bigdeli, T.B., Kretzschmar, W., Li, Y., Liang, J., Song, L., Hu, J., Li, Q., Jin, W., Hu, Z., 2015a. Sparse wholegenome sequencing identifies two loci for major depressive disorder. Nature 523 (7562), 588e591. Cai, N., Chang, S., Li, Y., Li, Q., Hu, J., Liang, J., Song, L., Kretzschmar, W., Gan, X., Nicod, J., Rivera, M., Deng, H., Du, B., Li, K., Sang, W., Gao, J., Gao, S., Ha, B., Ho, H.Y., Hu, C., Hu, J., Hu, Z., Huang, G., Jiang, G., Jiang, T., Jin, W., Li, G., Li, K., Li, Y., Li, Y., Li, Y., Lin, Y.T., Liu, L., Liu, T., Liu, Y., Liu, Y., Lu, Y., Lv, L., Meng, H., Qian, P., Sang, H., Shen, J., Shi, J., Sun, J., Tao, M., Wang, G., Wang, G., Wang, J., Wang, L., Wang, X., Wang, X., Yang, H., Yang, L., Yin, Y., Zhang, J., Zhang, K., Sun, N., Zhang, W., Zhang, X., Zhang, Z., Zhong, H., Breen, G., Wang, J., Marchini, J., Chen, Y., Xu, Q., Xu, X., Mott, R., Huang, G.J., Kendler, K., Flint, J., 2015b. Molecular signatures of major depression. Current Biology 25 (9), 1146e1156. Castro, J.E., Diessler, S., Varea, E., Marquez, C., Larsen, M.H., Cordero, M.I., Sandi, C., 2012. Personality traits in rats predict vulnerability and resilience to developing stress-induced depression-like behaviors, HPA axis hyperreactivity and brain changes in pERK1/2 activity. Psychoneuroendocrinology 37 (8), 1209e1223. Czarny, P., Wigner, P., Galecki, P., Sliwinski, T., 2018. The interplay between inflammation, oxidative stress, DNA damage, DNA repair and mitochondrial dysfunction in depression. Progress in Neuro-Psychopharmacology and Biological Psychiatry 80 (Pt C), 309e321. Delaney, K.R., Tank, D.W., 1994. A quantitative measurement of the dependence of short-term synaptic enhancement on presynaptic residual calcium. Journal of Neuroscience 14 (10), 5885e5902. DiMauro, S., Schon, E.A., Carelli, V., Hirano, M., 2013. The clinical maze of mitochondrial neurology. Nature Reviews Neurology 9 (8), 429e444. Du, J., Wang, Y., Hunter, R., Wei, Y., Blumenthal, R., Falke, C., Khairova, R., Zhou, R., Yuan, P., Machado-Vieira, R., McEwen, B.S., Manji, H.K., 2009. Dynamic regulation of mitochondrial function by glucocorticoids. Proceedings of the National Academy of Sciences of the United States of America 106 (9), 3543e3548. Duclot, F., Kabbaj, M., 2013. Individual differences in novelty seeking predict subsequent vulnerability to social defeat through a differential epigenetic regulation of brain-derived neurotrophic factor expression. Journal of Neuroscience 33 (27), 11048e11060. Fan, X., Li, D., Lichti, C.F., Green, T.A., 2013. Dynamic proteomics of nucleus accumbens in response to acute psychological stress in environmentally enriched and isolated rats. PLoS One 8 (9), e73689. Fattal, O., Link, J., Quinn, K., Cohen, B.H., Franco, K., 2007. Psychiatric comorbidity in 36 adults with mitochondrial cytopathies. CNS Spectrums 12 (6), 429e438. Filiou, M.D., Zhang, Y., Teplytska, L., Reckow, S., Gormanns, P., Maccarrone, G., Frank, E., Kessler, M.S., Hambsch, B., Nussbaumer, M., Bunck, M., Ludwig, T., Yassouridis, A., Holsboer, F., Landgraf, R., Turck, C.W., 2011. Proteomics and metabolomics analysis of a trait anxiety mouse model reveals divergent mitochondrial pathways. Biological Psychiatry 70 (11), 1074e1082. Flaquer, A., Baumbach, C., Ladwig, K.H., Kriebel, J., Waldenberger, M., Grallert, H., Baumert, J., Meitinger, T., Kruse, J., Peters, A., Emeny, R., Strauch, K., 2015. Mitochondrial genetic variants identified to be associated with posttraumatic stress disorder. Translational Psychiatry 5, e524. Ge, J.F., Peng, L., Cheng, J.Q., Pan, C.X., Tang, J., Chen, F.H., Li, J., 2013. Antidepressant-like effect of resveratrol: involvement of antioxidant effect and peripheral regulation on HPA axis. Pharmacology Biochemistry and Behavior 114e115, 64e69. Gong, Y., Chai, Y., Ding, J.H., Sun, X.L., Hu, G., 2011. Chronic mild stress damages mitochondrial ultrastructure and function in mouse brain. Neuroscience Letters 488 (1), 76e80. Gottschalk, M.G., Wesseling, H., Guest, P.C., Bahn, S., 2014. Proteomic enrichment analysis of psychotic and affective disorders reveals common signatures in presynaptic glutamatergic signaling and energy metabolism. The International Journal of Neuropsychopharmacology 18 (2). Harris, J.J., Jolivet, R., Attwell, D., 2012. Synaptic energy use and supply. Neuron 75 (5), 762e777. Hollis, F., van der Kooij, M.A., Zanoletti, O., Lozano, L., Canto, C., Sandi, C., 2015. Mitochondrial function in the brain links anxiety with social subordination. Proceedings of the National Academy of Sciences of the United States of America 112 (50), 15486e15491. Houtkooper, R.H., Pirinen, E., Auwerx, J., 2012. Sirtuins as regulators of metabolism and healthspan. Nature Reviews Molecular Cell Biology 13 (4), 225e238. Hunter, R.G., Seligsohn, M., Rubin, T.G., Griffiths, B.B., Ozdemir, Y., Pfaff, D.W., Datson, N.A., McEwen, B.S., 2016. Stress and corticosteroids regulate rat hippocampal mitochondrial DNA gene expression via the glucocorticoid receptor. Proceedings of the National Academy of Sciences of the United States of America 113 (32), 9099e9104.

References

129

Im, J.J., Namgung, E., Choi, Y., Kim, J.Y., Rhie, S.J., Yoon, S., 2016. Molecular neuroimaging in posttraumatic stress disorder. Experimental Neurobiology 25 (6), 277e295. Inczedy-Farkas, G., Remenyi, V., Gal, A., Varga, Z., Balla, P., Udvardy-Meszaros, A., Bereznai, B., Molnar, M.J., 2012. Psychiatric symptoms of patients with primary mitochondrial DNA disorders. Behavioral and Brain Functions 8, 9. Johnston-Wilson, N.L., Sims, C.D., Hofmann, J.P., Anderson, L., Shore, A.D., Torrey, E.F., Yolken, R.H., 2000. Disease-specific alterations in frontal cortex brain proteins in schizophrenia, bipolar disorder, and major depressive disorder. The Stanley Neuropathology Consortium. Molecular Psychiatry 5 (2), 142e149. Kamer, K.J., Mootha, V.K., 2015. The molecular era of the mitochondrial calcium uniporter. Nature Reviews Molecular Cell Biology 16 (9), 545e553. Kang, J.S., Tian, J.H., Pan, P.Y., Zald, P., Li, C., Deng, C., Sheng, Z.H., 2008. Docking of axonal mitochondria by syntaphilin controls their mobility and affects short-term facilitation. Cell 132 (1), 137e148. Karabatsiakis, A., Bock, C., Salinas-Manrique, J., Kolassa, S., Calzia, E., Dietrich, D.E., Kolassa, I.T., 2014. Mitochondrial respiration in peripheral blood mononuclear cells correlates with depressive subsymptoms and severity of major depression. Translational Psychiatry 4, e397. Kasahara, T., Takata, A., Kato, T.M., Kubota-Sakashita, M., Sawada, T., Kakita, A., Mizukami, H., Kaneda, D., Ozawa, K., Kato, T., 2016. Depression-like episodes in mice harboring mtDNA deletions in paraventricular thalamus. Molecular Psychiatry 21 (1), 39e48. Kim, H.D., Hesterman, J., Call, T., Magazu, S., Keeley, E., Armenta, K., Kronman, H., Neve, R.L., Nestler, E.J., Ferguson, D., 2016. SIRT1 mediates depression-like behaviors in the nucleus accumbens. Journal of Neuroscience 36 (32), 8441e8452. Kishi, T., Yoshimura, R., Kitajima, T., Okochi, T., Okumura, T., Tsunoka, T., Yamanouchi, Y., Kinoshita, Y., Kawashima, K., Fukuo, Y., Naitoh, H., Umene-Nakano, W., Inada, T., Nakamura, J., Ozaki, N., Iwata, N., 2010. SIRT1 gene is associated with major depressive disorder in the Japanese population. Journal of Affective Disorders 126 (1e2), 167e173. Klok, M.D., Giltay, E.J., Van der Does, A.J., Geleijnse, J.M., Antypa, N., Penninx, B.W., de Geus, E.J., Willemsen, G., Boomsma, D.I., van Leeuwen, N., Zitman, F.G., de Kloet, E.R., DeRijk, R.H., 2011. A common and functional mineralocorticoid receptor haplotype enhances optimism and protects against depression in females. Translational Psychiatry 1, e62. Koene, S., Kozicz, T.L., Rodenburg, R.J., Verhaak, C.M., de Vries, M.C., Wortmann, S., van de Heuvel, L., Smeitink, J.A., Morava, E., 2009. Major depression in adolescent children consecutively diagnosed with mitochondrial disorder. Journal of Affective Disorders 114 (1e3), 327e332. Kondo, D.G., Sung, Y.H., Hellem, T.L., Fiedler, K.K., Shi, X., Jeong, E.K., Renshaw, P.F., 2011. Open-label adjunctive creatine for female adolescents with SSRI-resistant major depressive disorder: a 31-phosphorus magnetic resonance spectroscopy study. Journal of Affective Disorders 135 (1e3), 354e361. Kovanen, L., Donner, K., Partonen, T., 2015. SIRT1 polymorphisms associate with seasonal weight variation, depressive disorders, and diastolic blood pressure in the general population. PLoS One 10 (10), e0141001. Larrieu, T., Cherix, A., Duque, A., Rodrigues, J., Lei, H., Gruetter, R., Sandi, C., 2017. Hierarchical status predicts behavioral vulnerability and nucleus accumbens metabolic profile following chronic social defeat stress. Current Biology 27 (14), 2202e2210 e2204. Levy, M., Faas, G.C., Saggau, P., Craigen, W.J., Sweatt, J.D., 2003. Mitochondrial regulation of synaptic plasticity in the hippocampus. Journal of Biological Chemistry 278 (20), 17727e17734. Li, H., Li, X., Smerin, S.E., Zhang, L., Jia, M., Xing, G., Su, Y.A., Wen, J., Benedek, D., Ursano, R., 2014. Mitochondrial gene expression profiles and metabolic pathways in the amygdala associated with exaggerated fear in an animal model of PTSD. Frontiers in Neurology 5, 164. Li, Z., Okamoto, K., Hayashi, Y., Sheng, M., 2004. The importance of dendritic mitochondria in the morphogenesis and plasticity of spines and synapses. Cell 119 (6), 873e887. Libert, S., Pointer, K., Bell, E.L., Das, A., Cohen, D.E., Asara, J.M., Kapur, K., Bergmann, S., Preisig, M., Otowa, T., Kendler, K.S., Chen, X., Hettema, J.M., van den Oord, E.J., Rubio, J.P., Guarente, L., 2011. SIRT1 activates MAO-A in the brain to mediate anxiety and exploratory drive. Cell 147 (7), 1459e1472. Liu, D., Xie, K., Yang, X., Gu, J., Ge, L., Wang, X., Wang, Z., 2014. Resveratrol reverses the effects of chronic unpredictable mild stress on behavior, serum corticosterone levels and BDNF expression in rats. Behavioural Brain Research 264, 9e16.

130

9. Mitochondrial function and stress resilience

Liu, J., Wang, X., Shigenaga, M.K., Yeo, H.C., Mori, A., Ames, B.N., 1996. Immobilization stress causes oxidative damage to lipid, protein, and DNA in the brain of rats. The FASEB Journal 10 (13), 1532e1538. Liu, T., Zhong, S., Liao, X., Chen, J., He, T., Lai, S., Jia, Y., 2015. A meta-analysis of oxidative stress markers in depression. PLoS One 10 (10), e0138904. Lo Iacono, L., Visco-Comandini, F., Valzania, A., Viscomi, M.T., Coviello, M., Giampa, A., Roscini, L., Bisicchia, E., Siracusano, A., Troisi, A., Puglisi-Allegra, S., Carola, V., 2015. Adversity in childhood and depression: linked through SIRT1. Translational Psychiatry 5, e629. Lord, L.D., Expert, P., Huckins, J.F., Turkheimer, F.E., 2013. Cerebral energy metabolism and the brain’s functional network architecture: an integrative review. Journal of Cerebral Blood Flow and Metabolism 33 (9), 1347e1354. Lucca, G., Comim, C.M., Valvassori, S.S., Reus, G.Z., Vuolo, F., Petronilho, F., Gavioli, E.C., Dal-Pizzol, F., Quevedo, J., 2009. Increased oxidative stress in submitochondrial particles into the brain of rats submitted to the chronic mild stress paradigm. Journal of Psychiatric Research 43 (9), 864e869. Luksys, G., Sandi, C., 2011. Neural mechanisms and computations underlying stress effects on learning and memory. Current Opinion in Neurobiology 21 (3), 502e508. Lyoo, I.K., Yoon, S., Kim, T.S., Hwang, J., Kim, J.E., Won, W., Bae, S., Renshaw, P.F., 2012. A randomized, doubleblind placebo-controlled trial of oral creatine monohydrate augmentation for enhanced response to a selective serotonin reuptake inhibitor in women with major depressive disorder. American Journal of Psychiatry 169 (9), 937e945. Manji, H., Kato, T., Di Prospero, N.A., Ness, S., Beal, M.F., Krams, M., Chen, G., 2012. Impaired mitochondrial function in psychiatric disorders. Nature Reviews Neuroscience 13 (5), 293e307. Marland, J.R., Hasel, P., Bonnycastle, K., Cousin, M.A., 2016. Mitochondrial calcium uptake modulates synaptic vesicle endocytosis in central nerve terminals. Journal of Biological Chemistry 291 (5), 2080e2086. Martins-de-Souza, D., Guest, P.C., Harris, L.W., Vanattou-Saifoudine, N., Webster, M.J., Rahmoune, H., Bahn, S., 2012. Identification of proteomic signatures associated with depression and psychotic depression in postmortem brains from major depression patients. Translational Psychiatry 2, e87. McEwen, B.S., Bowles, N.P., Gray, J.D., Hill, M.N., Hunter, R.G., Karatsoreos, I.N., Nasca, C., 2015. Mechanisms of stress in the brain. Nature Neuroscience 18 (10), 1353e1363. McEwen, B.S., Chattarji, S., 2004. Molecular mechanisms of neuroplasticity and pharmacological implications: the example of tianeptine. European Neuropsychopharmacology 14 (Suppl. 5), S497eS502. Miller, M.W., Sadeh, N., 2014. Traumatic stress, oxidative stress and post-traumatic stress disorder: neurodegeneration and the accelerated-aging hypothesis. Molecular Psychiatry 19 (11), 1156e1162. Miller, M.W., Wolf, E.J., Sadeh, N., Logue, M., Spielberg, J.M., Hayes, J.P., Sperbeck, E., Schichman, S.A., Stone, A., Carter, W.C., Humphries, D.E., Milberg, W., McGlinchey, R., 2015. A novel locus in the oxidative stress-related gene ALOX12 moderates the association between PTSD and thickness of the prefrontal cortex. Psychoneuroendocrinology 62, 359e365. Mink, J.W., Blumenschine, R.J., Adams, D.B., 1981. Ratio of central nervous system to body metabolism in vertebrates: its constancy and functional basis. American Journal of Physiology 241 (3), R203eR212. Moore, C.M., Christensen, J.D., Lafer, B., Fava, M., Renshaw, P.F., 1997. Lower levels of nucleoside triphosphate in the basal ganglia of depressed subjects: a phosphorous-31 magnetic resonance spectroscopy study. American Journal of Psychiatry 154 (1), 116e118. Morava, E., Gardeitchik, T., Kozicz, T., de Boer, L., Koene, S., de Vries, M.C., McFarland, R., Roobol, T., Rodenburg, R.J., Verhaak, C.M., 2010. Depressive behaviour in children diagnosed with a mitochondrial disorder. Mitochondrion 10 (5), 528e533. Morava, E., Kozicz, T., 2013. Mitochondria and the economy of stress (mal)adaptation. Neuroscience and Biobehavioral Reviews 37 (4), 668e680. Murphy, M.P., 2009. How mitochondria produce reactive oxygen species. Biochemical Journal 417 (1), 1e13. Nemets, B., Levine, J., 2013. A pilot dose-finding clinical trial of creatine monohydrate augmentation to SSRIs/ SNRIs/NASA antidepressant treatment in major depression. International Clinical Psychopharmacology 28 (3), 127e133. Nievergelt, C.M., Maihofer, A.X., Mustapic, M., Yurgil, K.A., Schork, N.J., Miller, M.W., Logue, M.W., Geyer, M.A., Risbrough, V.B., O’Connor, D.T., Baker, D.G., 2015. Genomic predictors of combat stress vulnerability and resilience in U.S. Marines: a genome-wide association study across multiple ancestries implicates PRTFDC1 as a potential PTSD gene. Psychoneuroendocrinology 51, 459e471.

References

131

Nussbaumer, M., Asara, J.M., Teplytska, L., Murphy, M.P., Logan, A., Turck, C.W., Filiou, M.D., 2016. Selective mitochondrial targeting exerts anxiolytic effects in vivo. Neuropsychopharmacology 41 (7), 1751e1758. Palty, R., Hershfinkel, M., Sekler, I., 2012. Molecular identity and functional properties of the mitochondrial Naþ/ Ca2þ exchanger. Journal of Biological Chemistry 287 (38), 31650e31657. Pathak, D., Shields, L.Y., Mendelsohn, B.A., Haddad, D., Lin, W., Gerencser, A.A., Kim, H., Brand, M.D., Edwards, R.H., Nakamura, K., 2015. The role of mitochondrially derived ATP in synaptic vesicle recycling. Journal of Biological Chemistry 290 (37), 22325e22336. Picard, M., Juster, R.P., McEwen, B.S., 2014. Mitochondrial allostatic load puts the ‘gluc’ back in glucocorticoids. Nature Reviews Endocrinology 10 (5), 303e310. Picard, M., McManus, M.J., Gray, J.D., Nasca, C., Moffat, C., Kopinski, P.K., Seifert, E.L., McEwen, B.S., Wallace, D.C., 2015. Mitochondrial functions modulate neuroendocrine, metabolic, inflammatory, and transcriptional responses to acute psychological stress. Proceedings of the National Academy of Sciences of the United States of America 112 (48), E6614eE6623. Psarra, A.M., Sekeris, C.E., 2011. Glucocorticoids induce mitochondrial gene transcription in HepG2 cells: role of the mitochondrial glucocorticoid receptor. Biochimica et Biophysica Acta 1813 (10), 1814e1821. Rangaraju, V., Calloway, N., Ryan, T.A., 2014. Activity-driven local ATP synthesis is required for synaptic function. Cell 156 (4), 825e835. Reeve, A.K., Krishnan, K.J., Duchen, M.R., Turnbull, D.M., 2011. Mitochondrial Dysfunction in Neurodegenerative Disorders. Springer Science & Business Media. Rezin, G.T., Cardoso, M.R., Goncalves, C.L., Scaini, G., Fraga, D.B., Riegel, R.E., Comim, C.M., Quevedo, J., Streck, E.L., 2008. Inhibition of mitochondrial respiratory chain in brain of rats subjected to an experimental model of depression. Neurochemistry International 53 (6e8), 395e400. Russo, S.J., Murrough, J.W., Han, M.H., Charney, D.S., Nestler, E.J., 2012. Neurobiology of resilience. Nature Neuroscience 15 (11), 1475e1484. Sandi, C., Cordero, M.I., Ugolini, A., Varea, E., Caberlotto, L., Large, C.H., 2008. Chronic stress-induced alterations in amygdala responsiveness and behavior–modulation by trait anxiety and corticotropin-releasing factor systems. European Journal of Neuroscience 28 (9), 1836e1848. Sandi, C., Richter-Levin, G., 2009. From high anxiety trait to depression: a neurocognitive hypothesis. Trends in Neurosciences 32 (6), 312e320. Schieber, M., Chandel, N.S., 2014. ROS function in redox signaling and oxidative stress. Current Biology 24 (10), R453eR462. Shao, W.H., Chen, J.J., Fan, S.H., Lei, Y., Xu, H.B., Zhou, J., Cheng, P.F., Yang, Y.T., Rao, C.L., Wu, B., Liu, H.P., Xie, P., 2015. Combined metabolomics and proteomics analysis of major depression in an animal model: perturbed energy metabolism in the chronic mild stressed rat cerebellum. OMICS 19 (7), 383e392. Sharma, S., Powers, A., Bradley, B., Ressler, K.J., 2016. Gene x environment determinants of stress- and anxietyrelated disorders. Annual Review of Psychology 67, 239e261. Sharpley, M.S., Marciniak, C., Eckel-Mahan, K., McManus, M., Crimi, M., Waymire, K., Lin, C.S., Masubuchi, S., Friend, N., Koike, M., Chalkia, D., MacGregor, G., Sassone-Corsi, P., Wallace, D.C., 2012. Heteroplasmy of mouse mtDNA is genetically unstable and results in altered behavior and cognition. Cell 151 (2), 333e343. Sibson, N.R., Dhankhar, A., Mason, G.F., Rothman, D.L., Behar, K.L., Shulman, R.G., 1998. Stoichiometric coupling of brain glucose metabolism and glutamatergic neuronal activity. Proceedings of the National Academy of Sciences of the United States of America 95 (1), 316e321. Stanton, P.K., Schanne, F.A., 1986. Hippocampal long-term potentiation increases mitochondrial calcium pump activity in rat. Brain Research 382 (1), 185e188. Su, L., Cai, Y., Xu, Y., Dutt, A., Shi, S., Bramon, E., 2014. Cerebral metabolism in major depressive disorder: a voxelbased meta-analysis of positron emission tomography studies. BMC Psychiatry 14, 321. Su, Y.A., Wu, J., Zhang, L., Zhang, Q., Su, D.M., He, P., Wang, B.D., Li, H., Webster, M.J., Traumatic Stress Brain Study, G., Rennert, O.M., Ursano, R.J., 2008. Dysregulated mitochondrial genes and networks with drug targets in postmortem brain of patients with posttraumatic stress disorder (PTSD) revealed by human mitochondriafocused cDNA microarrays. International Journal of Biological Sciences 4 (4), 223e235. Sun, T., Qiao, H., Pan, P.Y., Chen, Y., Sheng, Z.H., 2013. Motile axonal mitochondria contribute to the variability of presynaptic strength. Cell Reports 4 (3), 413e419.

132

9. Mitochondrial function and stress resilience

Tang, Y., Zucker, R.S., 1997. Mitochondrial involvement in post-tetanic potentiation of synaptic transmission. Neuron 18 (3), 483e491. van der Kooij, M.A., Hollis, F., Lozano, L., Zalachoras, I., Abad, S., Zanoletti, O., Grosse, J., Guillot de Suduiraut, I., Canto, C., Sandi, C., 2018. Diazepam actions in the VTA enhance social dominance and mitochondrial function in the nucleus accumbens by activation of dopamine D1 receptors. Molecular Psychiatry 23 (3), 569e578. Verstreken, P., Ly, C.V., Venken, K.J., Koh, T.W., Zhou, Y., Bellen, H.J., 2005. Synaptic mitochondria are critical for mobilization of reserve pool vesicles at Drosophila neuromuscular junctions. Neuron 47 (3), 365e378. Videbech, P., 2000. PET measurements of brain glucose metabolism and blood flow in major depressive disorder: a critical review. Acta Psychiatrica Scandinavica 101 (1), 11e20. Villa, R.F., Ferrari, F., Bagini, L., Gorini, A., Brunello, N., Tascedda, F., 2017. Mitochondrial energy metabolism of rat hippocampus after treatment with the antidepressants desipramine and fluoxetine. Neuropharmacology 121, 30e38. Waagepetersen, H.S., Sonnewald, U., Gegelashvili, G., Larsson, O.M., Schousboe, A., 2001. Metabolic distinction between vesicular and cytosolic GABA in cultured GABAergic neurons using 13C magnetic resonance spectroscopy. Journal of Neuroscience Research 63 (4), 347e355. Weckmann, K., Deery, M.J., Howard, J.A., Feret, R., Asara, J.M., Dethloff, F., Filiou, M.D., Iannace, J., Labermaier, C., Maccarrone, G., Webhofer, C., Teplytska, L., Lilley, K., Muller, M.B., Turck, C.W., 2017. Ketamine’s antidepressant effect is mediated by energy metabolism and antioxidant defense system. Scientific Reports 7 (1), 15788. Wong, M.L., Arcos-Burgos, M., Liu, S., Velez, J.I., Yu, C., Baune, B.T., Jawahar, M.C., Arolt, V., Dannlowski, U., Chuah, A., Huttley, G.A., Fogarty, R., Lewis, M.D., Bornstein, S.R., Licinio, J., 2017. The PHF21B gene is associated with major depression and modulates the stress response. Molecular Psychiatry 22 (7), 1015e1025. Yang, F., He, X.P., Russell, J., Lu, B., 2003. Ca2þ influx-independent synaptic potentiation mediated by mitochondrial Na(þ)-Ca2þ exchanger and protein kinase C. The Journal of Cell Biology 163 (3), 511e523. Yao, W., Zhang, J.C., Ishima, T., Dong, C., Yang, C., Ren, Q., Ma, M., Han, M., Wu, J., Suganuma, H., Ushida, Y., Yamamoto, M., Hashimoto, K., 2016. Role of Keap1-Nrf2 signaling in depression and dietary intake of glucoraphanin confers stress resilience in mice. Scientific Reports 6, 30659. Yarana, C., Sanit, J., Chattipakorn, N., Chattipakorn, S., 2012. Synaptic and nonsynaptic mitochondria demonstrate a different degree of calcium-induced mitochondrial dysfunction. Life Sciences 90 (19e20), 808e814. Yehuda, R., Hoge, C.W., McFarlane, A.C., Vermetten, E., Lanius, R.A., Nievergelt, C.M., Hobfoll, S.E., Koenen, K.C., Neylan, T.C., Hyman, S.E., 2015. Post-traumatic stress disorder. Nature Reviews Disease Primers 1, 15057. Zhang, L., Li, H., Hu, X., Benedek, D.M., Fullerton, C.S., Forsten, R.D., Naifeh, J.A., Li, X., Wu, H., Benevides, K.N., Le, T., Smerin, S., Russell, D.W., Ursano, R.J., 2015. Mitochondria-focused gene expression profile reveals common pathways and CPT1B dysregulation in both rodent stress model and human subjects with PTSD. Translational Psychiatry 5, e580. Zuccoli, G.S., Saia-Cereda, V.M., Nascimento, J.M., Martins-de-Souza, D., 2017. The energy metabolism dysfunction in psychiatric disorders postmortem brains: focus on proteomic evidence. Frontiers in Neuroscience 11, 493.

C H A P T E R

10

Understanding resilience: biological approaches in at-risk populations A.V. Seligowski, S.B. Hill, C.D. King, A.P. Wingo, K.J. Ressler Division of Depression and Anxiety, McLean Hospital; Department of Psychiatry, Harvard Medical School, Belmont, MA, United States

Introduction Most of the world’s population experiences a traumatic event at some point in their lives. Although approximately 8% develop posttraumatic stress disorder (PTSD; Kessler et al., 2012), the large majority of individuals do not. Rather, many individuals who experience trauma have some initial symptoms, such as intrusive memories or avoidance of reminders (e.g., places that resemble where the trauma occurred), which dissipate within weeks. A large body of research is dedicated to understanding what places individuals at greater risk for developing PTSD and related functional impairment. An additional, important area of research focuses on the individuals who do not develop significant psychopathology. These individuals are typically referred to as being “resilient.” There are different ways to define resilience; however, the field has not solidified the proper way to understand it. Some define resilience as the lack of PTSD or other pathology, despite the presence of risk/trauma exposure. This may be captured by self-report and interview measures as a below-threshold score (i.e., below a clinical cutoff of PTSD). More nuanced conceptualizations of resilience suggest that it reflects more active and adaptive processes that help individuals recover from stressful and traumatic events (Charney, 2004). Such perspectives suggest that resilience could even be considered a preexisting personality trait, independent of risk exposure. Thus, in addition to a lack of significant psychological symptoms, we can also define resilience by the specific mechanisms that help to reduce one’s risk of developing such symptoms. Although there is no single predictor of PTSD development following trauma, there do appear to be biological markers, mechanisms, and processes that contribute to the buffering of trauma’s effects.

Stress Resilience https://doi.org/10.1016/B978-0-12-813983-7.00010-0

133

Copyright © 2020 Elsevier Inc. All rights reserved.

134

10. Understanding resilience: biological approaches in at-risk populations

This chapter will discuss approaches for studying these biological facets of resilience as a trait in at-risk populations. Specifically, we will cover genetic, physiological, and neuroimaging approaches to the study of psychological resilience and discuss how resilience itself may be seen as an individual trait with its own biomarkers and intermediate phenotypes. A further understanding of resilience will also help inform therapeutic approaches aimed at enhancing, building, or training resilience in at-risk populations.

Definitions and measurement of resilience Resilience is a multidimensional construct, and its conceptualization has included a variety of elements ranging from personal characteristics to environmental factors. Some salient individual resilience attributes include ego strength, hardiness, positive emotions, optimism, spirituality/faith, adaptive coping styles, and cognitive flexibility (Feder et al., 2009; Southwick et al., 2005). Environmental factors contributing to resilience comprise role models, close and nurturing family bonds, and access to quality or supportive relationships (Feder et al., 2009; Southwick et al., 2005). Given its complexity, resilience has been operationally defined in various ways, and its measurement has been challenging. In a recent review by Windle et al. (2011), 15 measures of resilience were evaluated based on their psychometric properties. Although they did not identify a measure that could be considered a “gold standard” for assessing resilience, three scales stood out as having the highest overall ratings: The Connor-Davidson Resilience Scale (CDRISC), the Resilience Scale for Adults (RSA), and the Brief Resilience Scale (BRS). The CDRISC (Fig. 10.1) captures the core personality characteristics of resilience including hardiness, tenacity, strong self-efficacy, emotional and cognitive control under pressure,

FIGURE 10.1 Dimensions of resilience captured by the Connor-Davidson Resilience Scale (CDRISC).

Definitions and measurement of resilience

135

adaptability, ability to bounce back, spiritual coping, tolerance of negative affect, and goal orientation (Campbell-Sills and Stein, 2007; Connor and Davidson, 2003). It is a 25-item measure in which each item is rated on a scale of 0e4, with higher scores reflecting greater levels of resilience. The CDRISC is one of the most widely used and best validated measures of resilience (e.g., Campbell-Sills and Stein, 2007; Fincham et al., 2009; Green et al., 2010; Pietrzak et al., 2010; Stein et al., 2009; Wingo et al., 2010). It has been tested in the US general population, community samples, primary care patients, psychiatric patients, members of different ethnic groups and cultures (China, Korea, Japan, Pakistan, Iran, Portugal, Spain, Russia, the Netherlands, Australia, Italy, Norway, South Africa, Uganda, Gaza), survivors of various traumas, Alzheimer’s caregivers, adolescents, college students, adults, elders, selected professional or athletic groups, patients in treatment for PTSD, and Iraq combat veterans (see bibliography at www.cd-risc.com). It has excellent psychometric properties with an internal consistency Cronbach’s a of 0.85 and test-retest reliability correlation of 0.87 (Campbell-Sills and Stein, 2007; Connor and Davidson, 2003). The RSA is a 45-item measure of resilience covering five different dimensions: personal competence, social competence, family coherence, social support, and personal structure (Friborg et al., 2003). It has been used with both psychiatric outpatients and healthy control participants. The BRS is a six-item measure in which participants are asked to rate items on a scale of 1e5, with higher scores indicating greater resilience (Smith et al., 2008). The BRS has been tested among samples of undergraduate students, cardiac rehabilitation patients, patients with fibromyalgia, and healthy controls. Although not a direct measure of resilience, the Positive and Negative Affect Schedule (PANAS) may also lend itself to studies of resilience in the aftermath of trauma (Watson et al., 1988). The PANAS is a 20-item measure consisting of two mood scales for measuring positive and negative affect. The positive affect subscale assesses enthusiasm, determination, excitement, and interest, among other affective states. Positive affect has been theorized to promote flexible thinking, facilitate adaptive coping strategies, and counteract the physiological effects of negative emotions (Ong et al., 2009). Under stressful conditions, positive emotions are thought to sustain continued coping efforts and restore the vital resources that are depleted by stress (Feder et al., 2009; Ong et al., 2009). In a study of widows, positive emotions contributed to faster psychological recovery from the death of a spouse or life partner (Ong et al., 2009). Not surprisingly, individuals with high CDRISC scores tend to bias toward positive emotions when faced with uncertain emotional expressions (Arce et al., 2009). In short, daily experience of positive emotions helps individuals bounce back from major life stressors and increase resilience and life satisfaction (Cohn et al., 2009). Another measure relevant to the study of resilience is the Short Grit Scale (Grit-S; Duckworth and Quinn, 2009). Grit has been defined by Duckworth et al. (2007) as “perseverance and passion for long-term goals.” This involves continuously working toward goals and maintaining interest despite challenges. The Grit-S is an eight-item measure of grit consisting of two factors: Consistency of Interest (e.g., “I often set a goal but later choose to pursue a different one”; reverse scored) and Perseverance of Effort (e.g., “Setbacks don’t discourage me”; Duckworth and Quinn, 2009). Across multiple samples, the Grit-S has demonstrated strong psychometric properties, as well as associations with achievement, conscientiousness, and career and marriage stability (Duckworth and Quinn, 2009; Eskreis-Winkler et al., 2014).

136

10. Understanding resilience: biological approaches in at-risk populations

These associations remained significant despite the inclusion of variables such as intelligence and demographic factors. Among low-income adolescents, higher levels of grit may be protective against engaging in maladaptive behaviors such as substance use and fighting (Guerrero et al., 2016). Given that trait-level grit is relevant to several functional outcomes, it may be worthy of consideration in the context of resilience to trauma.

Biological facets of resilience Genetics Recent studies in the field of genetics have identified several candidate genes that appear to be implicated in resilience. The literature on specific candidate genes should be interpreted with caution because of the potential presence of bias and low statistical power. The primary focus of current genetics research in PTSD and resilience is on large, unbiased genome-wide association studies (GWASs). We will review some of the more robust candidate gene studies and recent GWASs that relate to resilience. Candidate studies One particular gene that has received attention is the gene encoding the serotonin (5HT) transporter, 5HTT. Specifically, a polymorphism in the promoter region of this gene (5HTTLPR) has been implicated as a genetic risk factor for poor stress resilience, such that the short allele variant (S) is less efficient at the cellular level of serotonin transport than the long allele variant (Lesch et al., 1996). With the occurrence of stressful life events (including trauma), this polymorphism has been shown to confer greater risk for depression (e.g., Caspi et al., 2003; Kaufman et al., 2004; Zalsman et al., 2006), anxiety sensitivity (Stein et al., 2008), and trait anxiety/neuroticism (Schinka et al., 2004). Expanding upon this research, Stein et al. (2009) examined 5HTTLPR in direct relation to emotional resilience, which was measured with the CDRISC (Connor and Davidson, 2003). Consistent with prior research, the authors found that individuals with one to two copies of the S allele variant demonstrated lower resilience scores than those without the S allele, and they also showed that this relationship was linear, such that increase in S allele copies was associated with lower resilience. Odds ratio analyses suggested that for each S allele copy, an individual’s chance of being in a “low-resilient” category increased by 63%. It is noteworthy that this was the first study to demonstrate that resilience itself, measured as a trait, was significantly associated with its own genetic risk factor. This provides further support for examining psychopathology and risk by their underlying intermediate phenotypes rather than symptom presentation alone, consistent with NIMH’s research domain criteria (RDoC) initiative. Pituitary adenylate cyclaseeactivating polypeptide (PACAP) has been implicated in the stress response, and in particular, it has been found to regulate corticotropin-releasing factor (CRF) function. Building on prior PACAP research in rodents, Ressler et al. (2011) examined its relations with PTSD among humans with trauma exposure. Lower levels of PACAP38 (a PACAP peptide) were associated with lower PTSD symptoms among females, but not males. This effect was also observed when the individual symptom clusters of PTSD were tested,

Biological facets of resilience

137

and both analyses were replicated in an additional female sample. Further analyses suggested that a single-nucleotide polymorphism (SNP) in the estrogen receptor response element within the PAC1 receptor ADCYAP1R1, rs2267735, was significantly predictive of PTSD diagnosis among females, but not males. The rs2267735 CG and GG genotypes in particular were associated with less severe PTSD symptoms, whereas the CC genotype was associated with more severe symptoms (Ressler et al., 2011). Thus the G-allele carriers in these studies could be seen as resilient to the effects of severe trauma exposure. The dopamine receptor gene, DRD4, has previously demonstrated a moderating effect on the relationship between childhood stress and both internalizing and externalizing behaviors among children. To explore its role in resilience, DRD4 was recently studied as a potential candidate gene among individuals with varying levels of child adversity. Das et al. (2011) tested the potential moderating effect of a specific polymorphism, DRD4-exIII-VNTR, on the relationship between child adversity and adult self-reported resilience. A gene x environment interaction was observed such that the 7rþ allele of DRD4-exIII-VNTR predicted greater resilience despite the presence of child adversity (as child adversity increased, resilience decreased among those with the 7r allele). Another dopaminergic gene that has received attention is the catechol-O-methyltransferase (COMT) gene. Specifically, the Met allele of the COMT gene has been associated with greater risk for PTSD and depression following stressful and/or traumatic events (Boscarino et al., 2011; Kolassa et al., 2010). Given that COMT is responsible for dopamine regulation in the brain (which has a role in inhibition), one of the proposed mechanisms for the effect of the Met allele on PTSD is fear inhibition/safety learning (Norrholm et al., 2013). To further examine this, van Rooij et al. (2016) tested the interaction of childhood trauma and COMT on inhibition and resilience among adults. Inhibition was assessed via hippocampal activation during a Go/NoGo task, and resilience was assessed via selfreport. The Met allele of COMT was associated with decreased inhibition, whereas the Val allele was predictive of improved inhibitiondpotentially an intermediate phenotype of resilience. Furthermore, hippocampal activity was positively correlated with trait resilience, and it mediated the effect of childhood abuse on resilience among those with the Val allele. These findings suggest that increased hippocampal activation (i.e., inhibition) may be a mechanism through which the Val allele of COMT confers greater resilience among individuals with childhood trauma. Given the known impact of trauma and PTSD on the hypothalamic-pituitary-adrenal (HPA) axis, another candidate gene implicated in risk/resilience is FKBP5 due to its regulation of glucocorticoid receptor activity. Specifically, SNPs in the FKBP5 gene have been shown to predict PTSD symptoms among trauma-exposed individuals. In a study of adults with child abuse and adult-onset trauma, Binder et al. (2008) found that four FKBP5 SNPs interacted with child abuse to predict adult PTSD symptoms. Thus, differences in genes associated with HPA axis regulation may confer risk or resilience for PTSD through their role in glucocorticoid receptor activation (i.e., the presence of certain SNPs may alter the effects of childhood trauma on stress hormone regulation, and subsequently PTSD). A similar finding was reported by Sarapas et al. (2011), such that FKBP5 expression mediated the relationship between FKBP5 genotype and PTSD symptoms among adults exposed to the 9/11 attacks. The CRF receptor 1 gene (CRHR1) has also been studied in relation to resilience. Like FKBP5, CRHR1 is a gene involved in HPA axis regulation. Specifically, CRHR1 modulates

138

10. Understanding resilience: biological approaches in at-risk populations

the effect of CRF and the subsequent release of cortisol throughout the adrenal cortex. CRF is known to influence arousal, executive functioning, and memory consolidation. Several SNPs of CRHR1 have been shown to interact with childhood abuse to predict resilience, such that a TAT haplotype of three SNPs exerted a protective effect by reducing vulnerability to depression symptoms (Bradley et al., 2008). This finding was replicated by Polanczyk et al. (2009) in a sample of women who enduredchildhood maltreatment, such that the TAT haplotype was associated with reduced depression risk. The HPA axis is linked to the nitrous oxide network within the hippocampus (a brain region known to be involved in PTSD resilience due its role in memory consolidation). Given this link, two genes within the nitrous oxide pathway have been implicated in PTSD resilience: NOS1AP and NOS1 (Bruenig et al., 2017; Lawford et al., 2013). The proposed link between these systems involves N-methyl-D-aspartate (NMDA) and gamma-aminobutyric acid (GABA) activity. Specifically, changes in NMDA receptor activity (regulated by NOS1AP and NOS1) influence nitrous oxide production, and increased nitrous oxide contributes to lower GABA levels, which then leads to HPA axis dysfunction. In a study by Bruenig et al. (2017), a SNP of NOS1AP, rs4657178, was associated with selfreported resilience among trauma-exposed veterans without PTSD, whereas another SNP, rs17460657, was associated with resilience among those with PTSD. A SNP of NOS1, rs10744891, was also associated with resilience among the PTSD group. The SNP within the trauma-exposed control group may be of particular interest because it could highlight the role of NOS1AP SNP rs4657178 in promoting resilience from PTSD despite significant trauma exposure. In contrast, the findings within the PTSD group suggest that although rs17460657 and rs10744891 may foster greater resilience, they may not be protective against PTSD symptoms. Given its role in the stress response and fostering relationships, oxytocin is another candidate gene of interest. A recent study examined the rs53576 polymorphism of the oxytocin receptor gene (OXTR) in relation to resilient coping and positive affect among adults with varying degrees of trauma exposure (Bradley et al., 2013). For individuals with the GG and AG genotypes of OXTR rs53576, positive childhood environment was predictive of more resilient coping and positive affect in adulthood. In summary, a number of biological pathway-focused, candidate gene studies have demonstrated interesting potential genetic associations with resilience-related phenotypes. However, these studies have typically been underpowered, and the findings from these studies have not generally been replicated in larger-scale and unbiased studies. Thus, interpretations from these studies must be made with caution, with the need for replication and integration with larger-scale unbiased genome-wide studies and other biological complementary approaches, as outlined below. Genome-wide unbiased studies As part of a prospective longitudinal study of risk and resilience, Nievergelt et al. (2015) conducted a GWAS on US service members scheduled for overseas deployment. Across genetic ancestry groups, the phosphoribosyl transferase domain containing 1 gene (PRTFDC1) demonstrated genome-wide significance differentiating risk versus resilience responses to trauma exposure. This study was novel in its examination of effects across genetic ancestry, which also increased statistical power.

Biological facets of resilience

139

Other studies have approached resilience through the examination of transcriptome-wide analyses to identify which peripheral blood RNA expression may differentiate risk versus resilience. DICER1 is an enzyme that creates mature microRNAs (miRNAs) from premicroRNA. miRNAs are involved in the regulation of many genes and in synaptic development and plasticity. Recently, a series of studies implicated DICER1 and miRNAs in PTSD risk/resilience among adults with trauma exposure. In a transcriptome-wide association study, persons with comorbid PTSD and depression had significantly lower expression of DICER1 relative to controls after adjusting for the relevant confounding factors (genomewide false discovery rate at P < .05; Wingo et al., 2015). This association was replicated in two independent samples (Wingo et al., 2015). Lower DICER1 expression was also associated with greater amygdala activation in response to threat stimuli (an intermediate phenotype of PTSD). This finding provides a neurobiological parallel to that of the PTSD and depression diagnosis and provides further support for the potential role of DICER1 as a contributor to risk or resilience following trauma. Additionally, expression level of miR-3130-5p was significantly lower in persons with PTSD with comorbid depression compared with controls (Wingo et al., 2015). This association with risk versus resilience was replicated in an independent sample (Wingo et al., 2015). Since frequent experience of positive emotion despite high level of stress or trauma is a marker of resilience, Wingo et al. (2016) next conducted a GWAS of tendency to experience positive emotion among a high-risk sample of inner-city adults with high stress and trauma exposure. Two SNPs reached genome-wide significance, such that the minor alleles of rs322931 and rs7550394 were associated with more frequent experience of positive emotion. The minor allele of rs322931 was also significantly associated with better fear inhibition during a fear conditioning task. Wingo et al. (2016) suggested that this effect may be mediated by miR-181a and miR-181b given that rs322931 influences the expression of brain miR-181a. Prior studies have implicated miR-181a in reward neurocircuitry as well as synaptic plasticity, which are relevant to learning and resilience.

Physiology Distinctions in physiological responses between those with and without PTSD point to resilience factors that may be important following trauma exposure. In fear conditioning paradigms, individuals with PTSD have demonstrated decreased ability to distinguish between safety stimuli and danger stimuli compared with those without PTSD (i.e., decreased fear discrimination; Glover et al., 2011; Norrholm et al., 2011; Schumacher et al., 2013). Additionally, those with PTSD demonstrate higher startle to a safety cue (i.e., decreased fear inhibition; Jovanovic et al., 2009; Norrholm et al., 2011; Sijbrandij et al., 2013). Overall, this suggests that better fear discrimination and fear inhibition may contribute to psychological resilience following trauma. In a prospective longitudinal study of soldiers who were about to be deployed, similar results were found during the extinction phase of the conditioning paradigm. Specifically, better extinction learning prior to deployment was predictive of fewer PTSD symptoms (resilience) after soldiers returned from deployment (Lommen et al., 2013). Similarly, a study of police academy cadets found that those who reported less subjective fear under low threat had less severe symptoms of PTSD when assessed 1 year later (Pole et al., 2009).

140

10. Understanding resilience: biological approaches in at-risk populations

Skin conductance is another physiological measure that has been implicated in resilience following trauma exposure. In general, individuals with trauma exposure and no PTSD tend to demonstrate lower skin conductance than those with PTSD, suggesting that they experience less sympathetic arousal both at rest and in response to challenge (Pole, 2007). Heart rate variability (HRV), or variability in the time period between heart beats, is an autonomic nervous system indicator typically associated with emotion regulation. Higher HRV is associated with better emotion regulation (Beauchaine, 2001; Demaree et al., 2004; Volokhov and Demaree, 2010) and with fewer symptoms of PTSD among trauma-exposed individuals (e.g., Hauschildt et al., 2011; Sack et al., 2004; Shah et al., 2013). Thus, skin conductance and HRV may be considered additional resilience factors following trauma exposure, although no studies to date have examined these measures in relation to self-reported resilience. Although not exhaustive, Table 10.1 includes a summary of the studies implicating startle response, skin conductance, and HRV in recovery from trauma.

Neuroimaging Other predictors of resilience following trauma may be gleaned from structural and functional brain differences among individuals with and without PTSD. Given its role in learning and memory, the hippocampus has long been a region of interest related to PTSD development and maintenance. In general, individuals with PTSD tend to have smaller hippocampal volume (see Karl et al., 2006 and O’Doherty et al., 2015 for reviews). However, few studies have examined hippocampal volume in direct relation to resilience measured as a trait. Rather, most studies have compared hippocampal volume among trauma-exposed individuals with and without PTSD, where those without PTSD are considered “resilient.” Additionally, studies have been limited in their retrospective nature, such that it is often unclear whether volumetric differences were present before or after trauma exposure. In an effort to address these issues, Gilbertson et al. (2002) examined hippocampal volume among monozygotic twin pairs, where one twin was a veteran with combat exposure and the other was a nonveteran. Smaller right hippocampal volume was predictive of PTSD in trauma-exposed individuals. Notably, healthy identical twins of low-severity PTSD veterans showed significantly greater hippocampal volume than those with PTSD or their nonexposed siblings. Overall, this indicates that greater hippocampal volume may serve as a preexisting resilience factor for individuals exposed to trauma (Gilbertson et al., 2002). Building on this research, a recent study examined baseline hippocampal volume in relation to treatment response. In a study of individuals with PTSD who underwent prolonged exposure (PE), those who responded well to treatment (suggesting more resilience) had greater hippocampal volume at baseline than those who did not respond (Rubin et al., 2016). Two other brain regions of interest are the amygdala and the anterior cingulate cortex (ACC). Although the amygdala is involved in emotional memory and fear conditioning, the ACC plays a role in emotion regulation. Overall, individuals with PTSD tend to have smaller amygdala and ACC volumes compared with those without PTSD (see Karl et al., 2006 and O’Doherty et al., 2015 for reviews). Similar to studies of the hippocampus, amygdala and ACC research has primarily focused on comparisons between trauma-exposed

141

Biological facets of resilience

TABLE 10.1

Examples of biological facets of resilience.

Genes

Physiology

Neuroanatomy

5HTTLPR Stein et al. (2009)

Heart rate variability Green et al. (2016), Hauschildt et al. (2011), Keary et al. (2009), Liddell et al. (2016), Minassian et al. (2014), Park et al. (2017), Sack et al. (2004), Sahar et al. (2001), Shah et al. (2013)

Anterior cingulate cortex Chen et al. (2006, 2012), Kitayama et al. (2006), Felmingham et al. (2009) Reynaud et al. (2013), Rocha-Rego et al. (2012), Woodward et al. (2006)

COMT van Rooij et al. (2016)

Skin conductance Bryant et al. (1995), Carson et al. (2000), Casada et al. (1998), Cuthbert et al. (2003), Goldfinger et al. (1998), Hinrichs et al. (2017), Liberzon et al. (1999), Orr et al. (1995/2000), Pitman et al. (1990/2001), Pole et al. (2009), Rothbaum et al. (2001), Shalev et al. (1997)

Amygdala Morey et al. (2012), Rogers et al. (2009), Weniger et al. (2008), Wignall et al. (2004)

CRHR1 Polanczyk et al. (2009)

Startle response Glover et al. (2011), Grillon et al. (1998), Jovanovic et al. (2009/2010/2012),

Hippocampus Apfel et al. (2011), Bonne et al. (2008), Bossini et al. (2008), Bremner et al. (1995), Bremner et al. (1997, 2003), Chen et al. (2006), Emdad et al. (2006), Felmingham et al. (2009), Gilbertson et al. (2002), Lindauer et al. (2004, 2005), Pavic et al. (2007), Villarreal et al. (2002), Wang et al. (2010), Weniger et al. (2008), Wignall et al. (2004), Yehuda et al. (2007), Zhang et al. (2011)

ADCYAP1R1 Ressler et al. (2011)

DICER1 Wingo et al. (2015) DRD4 Das et al. (2011) FKBP5 Binder et al. (2008), Sarapas et al. (2011) miR-181a, miR-181b Wingo et al. (2016) NOS1, NOS1AP Bruenig et al. (2017) OXTR Bradley et al. (2013) PRTFDC1 Nievergelt et al. (2015)

Morgan et al. (1995/1996/1997), Orr et al. (1995), Norrholm et al. (2011), Schumacher et al. (2013), Shalev et al. (1998), Sijbrandij et al. (2013)

142

10. Understanding resilience: biological approaches in at-risk populations

individuals with and without PTSD rather than resilience as a trait, per se. As an example, one study explored resilience as a trait and found that increased amygdala activity was associated with greater self-reported resilience among fire-fighters (Reynaud et al., 2013). Given the known role of poor emotion regulation in PTSD, it follows that larger volumes/activation of brain regions implicated in emotion and its regulation may promote resilience following stressful life events. However, like research on the hippocampus, it is difficult to know whether increased amygdala and ACC volume/activation are protective factors or whether PTSD results in decreased amygdala/ACC volume/activation. Furthermore, it is unclear what differential gross measures of size or activation levels mean for neural circuit function. To further elucidate these findings, future neuroimaging studies would benefit from taking a prospective approach where possible (e.g., military and first-responder samples, monozygotic twin studies), as well as from examining resilience as a trait in addition to the lack of PTSD following trauma.

Resilience as a multidimensional trait Research on the aforementioned biological mechanisms of resilience provides support for the notion that resilience should be conceptualized as a trait with its own intermediate phenotypes rather than simply the lack of psychopathology. As a multidimensional trait, resilience encompasses differential heritability interacting with the environment, which leads to improved function and well-being via intermediate phenotypes such as improved regulation of emotion and physiology (Fig. 10.2). Future resilience research should also examine biological markers in association with specific resilience measures, such as the CDRISC. This approach would also allow for the probing of resilience in the context of PTSD. For example, some individuals may have a diagnosis of PTSD and yet many aspects of functioning could be intact. Examining resilience as a trait would allow researchers to better understand how some individuals cope with PTSD and are resilient despite significant symptoms. Taken together, the research reviewed here suggests that several candidate genes and SNPs, along with physiological mechanisms and neuroimaging characteristics, provide a biological model of resilience pathways.

FIGURE 10.2

Resilience as a multidimensional trait.

References

143

Conclusion/summary Despite the high prevalence of trauma exposure, most individuals do not develop significant symptoms of PTSD. It is important to better understand how so many trauma-exposed individuals do not develop significant symptoms, as well as how some have PTSD but are able to function despite these symptoms. Resilience research allows for the examination of factors that not only confer less risk for PTSD but also contribute to the resilience trait itself. Current measures for assessing resilience include the CDRISC, the RSA, the BRS, and the PANAS. Biological measures that are relevant to resilience following trauma include several genetic variants, brain regions, and physiological phenomena. Resilience studies may be improved by more incorporation of resilience-specific measures in addition to the study of group differences (PTSD vs. no-PTSD). Studying resilience as a trait can enhance future research and contribute to a better understanding of how individuals recover from trauma even in the face of PTSD symptoms.

Acknowledgments The work was supported by NIH grants R01MH108665, R01MH094757, and R21MH112956 and the Frazier Foundation Grant for Mood and Anxiety Research. APW is supported by grants IK2CX000601, R01AG056533, and U01HG009807.

Disclosures Dr. Ressler is on the Scientific Advisory Boards for Resilience Therapeutics, Sheppard Pratt-Lieber Research Institute, Laureate Institute for Brain Research, The Army STARRS Project, UCSD VA Center of Excellence for Stress and Mental HealthdCESAMH, and the Anxiety and Depression Association of America. He provides fee-for-service consultation for Biogen and Resilience Therapeutics. He holds patents for use of D-cycloserine and psychotherapy, targeting PAC1 receptor for extinction, targeting tachykinin 2 for prevention of fear, targeting angiotensin to improve extinction of fear. Dr. Ressler is also founding member of Extinction Pharmaceuticals to develop D-cycloserine to augment the effectiveness of psychotherapy, for which he has received no equity or income within the past 3 years. He receives or has received research funding from NIMH, HHMI, NARSAD, and the Burroughs Wellcome Foundation.

References Apfel, B.A., Ross, J., Hlavin, J., Meyerhoff, D.J., Metzler, T.J., Marmar, C.R., et al., 2011. Hippocampal volume differences in Gulf War veterans with current versus lifetime posttraumatic stress disorder symptoms. Biological Psychiatry 69, 541e548. Arce, E., Simmons, A.N., Stein, M.B., Winkielman, P., Hitchcock, C., Paulus, M.P., 2009. Association between individual differences in self-reported emotional resilience and the affective perception of neutral faces. Journal of Affective Disorders 114, 286e293. Beauchaine, T., 2001. Vagal tone, development, and Gray’s motivational theory: toward an integrated model of autonomic nervous system functioning in psychopathology. Development and Psychopathology 13, 183e214. Binder, E.B., Bradley, R.G., Liu, W., Epstein, M.P., Deveau, T.C., Mercer, K.B., et al., 2008. Association of FKBP5 polymorphisms and childhood abuse with risk of posttraumatic stress disorder symptoms in adults. Journal of the American Medical Association 299, 1291e1305. Bonne, O., Vythilingam, M., Inagaki, M., Wood, S., Neumeister, A., Nugent, A.C., et al., 2008. Reduced posterior hippocampal volume in posttraumatic stress disorder. The Journal of Clinical Psychiatry 69, 1087.

144

10. Understanding resilience: biological approaches in at-risk populations

Boscarino, J.A., Erlich, P.M., Hoffman, S.N., Rukstalis, M., Stewart, W.F., 2011. Association of FKBP5, COMT and CHRNA5 polymorphisms with PTSD among outpatients at risk for PTSD. Psychiatry Research 188, 173e174. Bossini, L., Tavanti, M., Calossi, S., Lombardelli, A., Polizzotto, N.R., Galli, R., et al., 2008. Magnetic resonance imaging volumes of the hippocampus in drug-naive patients with post-traumatic stress disorder without comorbidity conditions. Journal of Psychiatric Research 42, 752e762. Bradley, R., Binder, E.B., Epstein, M.P., Tang, Y., Nair, H.P., Liu, W., et al., 2008. Influence of child abuse on adult depression: moderation by the corticotropin-releasing hormone receptor gene. Archives of General Psychiatry 65, 190e200. Bradley, B., Davis, T.A., Wingo, A.P., Mercer, K.B., Ressler, K.J., 2013. Family environment and adult resilience: contributions of positive parenting and the oxytocin receptor gene. European Journal of Psychotraumatology 4, 21659. Bremner, J.D., Randall, P., Scott, T.M., Bronen, R.A., Seibyl, J.P., Southwick, S.M., et al., 1995. MRI-based measurement of hippocampal volume in patients with combat-related posttraumatic stress disorder. The American Journal of Psychiatry 152, 973. Bremner, J.D., Randall, P., Vermetten, E., Staib, L., Bronen, R.A., Mazure, C., et al., 1997. Magnetic resonance imagingbased measurement of hippocampal volume in posttraumatic stress disorder related to childhood physical and sexual abuseda preliminary report. Biological Psychiatry 41, 23e32. Bremner, J.D., Vythilingam, M., Vermetten, E., Southwick, S.M., McGlashan, T., Nazeer, A., et al., 2003. MRI and PET study of deficits in hippocampal structure and function in women with childhood sexual abuse and posttraumatic stress disorder. American Journal of Psychiatry 160, 924e932. Bruenig, D., Morris, C.P., Mehta, D., Harvey, W., Lawford, B., Young, R.M., Voisey, J., 2017. Nitric oxide pathway genes (NOS1AP and NOS1) are involved in PTSD severity, depression, anxiety, stress and resilience. Gene 625, 42e48. Bryant, R.A., Harvey, A.G., Gordon, E., Barry, R.J., 1995. Eye movement and electrodermal responses to threat stimuli in post-traumatic stress disorder. International Journal of Psychophysiology 20, 209e213. Campbell-Sills, L., Stein, M.B., 2007. Psychometric analysis and refinement of the ConnoreDavidson resilience scale (CD-RISC): validation of a 10-item measure of resilience. Journal of Traumatic Stress 20, 1019e1028. Carson, M.A., Paulus, L.A., Lasko, N.B., Metzger, L.J., Wolfe, J., Orr, S.P., Pitman, R.K., 2000. Psychophysiologic assessment of posttraumatic stress disorder in Vietnam nurse veterans who witnessed injury or death. Journal of Consulting and Clinical Psychology 68, 890. Casada, J.H., Amdur, R., Larsen, R., Liberzon, I., 1998. Psychophysiologic responsivity in posttraumatic stress disorder: generalized hyperresponsiveness versus trauma specificity. Biological Psychiatry 44, 1037e1044. Caspi, A., Sugden, K., Moffitt, T.E., Taylor, A., Craig, I.W., Harrington, H., et al., 2003. Influence of life stress on depression: moderation by a polymorphism in the 5-HTT gene. Science 301, 386e389. Charney, D.S., 2004. Psychobiological mechanisms of resilience and vulnerability: implications for successful adaptation to extreme stress. American Journal of Psychiatry 161, 195e216. Chen, Y., Fu, K., Feng, C., Tang, L., Zhang, J., Huan, Y., et al., 2012. Different regional gray matter loss in recent onset PTSD and non PTSD after a single prolonged trauma exposure. PLoS One 7, e48298. Chen, S., Xia, W., Li, L., Liu, J., He, Z., Zhang, Z., et al., 2006. Gray matter density reduction in the insula in fire survivors with posttraumatic stress disorder: a voxel-based morphometric study. Psychiatry Research: Neuroimaging 146, 65e72. Cohn, M.A., Fredrickson, B.L., Brown, S.L., Mikels, J.A., Conway, A.M., 2009. Happiness unpacked: positive emotions increase life satisfaction by building resilience. Emotion 9, 361e368. Connor, K.M., Davidson, J.R., 2003. Development of a new resilience scale: the Connor-Davidson resilience scale (CD-RISC). Depression and Anxiety 18, 76e82. Cuthbert, B.N., Lang, P.J., Strauss, C., Drobes, D., Patrick, C.J., Bradley, M.M., 2003. The psychophysiology of anxiety disorder: fear memory imagery. Psychophysiology 40, 407e422. Das, D., Cherbuin, N., Tan, X., Anstey, K.J., Easteal, S., 2011. DRD4-exonIII-VNTR moderates the effect of childhood adversities on emotional resilience in young adults. PLoS One 6. Demaree, H.A., Robinson, J.L., Erik Everhart, D., Schmeichel, B.J., 2004. Resting RSA is associated with natural and self-regulated responses to negative emotional stimuli. Brain and Cognition 56, 14e23. Duckworth, A.L., Peterson, C., Matthews, M.D., Kelly, D.R., 2007. Grit: perseverance and passion for long-term goals. Journal of Personality and Social Psychology 92, 1087.

References

145

Duckworth, A.L., Quinn, P.D., 2009. Development and validation of the short grit scale (griteS). Journal of Personality Assessment 91, 166e174. Emdad, R., Bonekamp, D., Söndergaard, H.P., Björklund, T., Agartz, I., Ingvar, M., Theorell, T., 2006. Morphometric and psychometric comparisons between non-substance-abusing patients with posttraumatic stress disorder and normal controls. Psychotherapy and Psychosomatics 75, 122e132. Eskreis-Winkler, L., Shulman, E.P., Beal, S.A., Duckworth, A.L., 2014. The grit effect: predicting retention in the military, the workplace, school and marriage. Frontiers in Psychology 5, 36. Feder, A., Nestler, E.J., Charney, D.S., 2009. Psychobiology and molecular genetics of resilience. Nature Reviews Neuroscience 10, 446e457. Felmingham, K.L., Williams, L.M., Kemp, A.H., Rennie, C., Gordon, E., Bryant, R.A., 2009. Anterior cingulate activity to salient stimuli is modulated by autonomic arousal in posttraumatic stress disorder. Psychiatry Research: Neuroimaging 173, 59e62. Fincham, D.S., Altes, L.K., Stein, D.J., Seedat, S., 2009. Posttraumatic stress disorder symptoms in adolescents: risk factors versus resilience moderation. Comprehensive Psychiatry 50, 193e199. Friborg, O., Hjemdal, O., Rosenvinge, J.H., Martinussen, M., 2003. A new rating scale for adult resilience: what are the central protective resources behind healthy adjustment? International Journal of Methods in Psychiatric Research 12, 65e76. Gilbertson, M.W., Shenton, M.E., Ciszewski, A., Kasai, K., Lasko, N.B., Orr, S.P., Pitman, R.K., 2002. Smaller hippocampal volume predicts pathologic vulnerability to psychological trauma. Nature Neuroscience 5, 1242e1247. Glover, E.M., Phifer, J.E., Crain, D.F., Norrholm, S.D., Davis, M., Bradley, B., et al., 2011. Tools for translational neuroscience: PTSD is associated with heightened fear responses using acoustic startle but not skin conductance measures. Depression and Anxiety 28, 1058e1066. Goldfinger, D.A., Amdur, R.L., Liberzon, I., 1998. Psychophysiologic responses to the Rorschach in PTSD patients, noncombat and combat controls. Depression and Anxiety 8, 112e120. Green, K.T., Calhoun, P.S., Dennis, M.F., Beckham, J.C., 2010. Exploration of the resilience construct in posttraumatic stress disorder severity and functional correlates in military combat veterans who have served since September 11, 2001. Journal of Clinical Psychiatry 71, 823e830. Green, K.T., Dennis, P.A., Neal, L.C., Hobkirk, A.L., Hicks, T.A., Watkins, L.L., et al., 2016. Exploring the relationship between posttraumatic stress disorder symptoms and momentary heart rate variability. Journal of Psychosomatic Research 82, 31e34. Grillon, C., Morgan III, C.A., Davis, M., Southwick, S.M., 1998. Effect of darkness on acoustic startle in Vietnam veterans with PTSD. American Journal of Psychiatry 155, 812e817. Guerrero, L.R., Dudovitz, R., Chung, P.J., Dosanjh, K.K., Wong, M.D., 2016. Grit: a potential protective factor against substance use and other risk behaviors among Latino adolescents. Academic Pediatrics 16, 275e281. Hauschildt, M., Peters, M.J., Moritz, S., Jelinek, L., 2011. Heart rate variability in response to affective scenes in posttraumatic stress disorder. Biological Psychology 88, 215e222. Hinrichs, R., Michopoulos, V., Winters, S., Rothbaum, A.O., Rothbaum, B.O., Ressler, K.J., Jovanovic, T., 2017. Mobile assessment of heightened skin conductance in posttraumatic stress disorder. Depression and Anxiety 34, 502e507. Jovanovic, T., Blanding, N.Q., Norrholm, S.D., Duncan, E., Bradley, B., Ressler, K.J., 2009. Childhood abuse is associated with increased startle reactivity in adulthood. Depression and Anxiety 26, 1018e1026. Jovanovic, T., Kazama, A., Bachevalier, J., Davis, M., 2012. Impaired safety signal learning may be a biomarker of PTSD. Neuropharmacology 62, 695e704. Jovanovic, T., Norrholm, S.D., Blanding, N.Q., Davis, M., Duncan, E., Bradley, B., Ressler, K.J., 2010. Impaired fear inhibition is a biomarker of PTSD but not depression. Depression and Anxiety 27, 244e251. Jovanovic, T., Norrholm, S.D., Fennell, J.E., Keyes, M., Fiallos, A.M., Myers, K.M., et al., 2009. Posttraumatic stress disorder may be associated with impaired fear inhibition: relation to symptom severity. Psychiatry Research 167, 151e160. Karl, A., Schaefer, M., Malta, L.S., Dörfel, D., Rohleder, N., Werner, A., 2006. A meta-analysis of structural brain abnormalities in PTSD. Neuroscience and Biobehavioral Reviews 30, 1004e1031. Kaufman, J., Yang, B.Z., Douglas-Palumberi, H., Houshyar, S., Lipschitz, D., Krystal, J.H., Gelernter, J., 2004. Social supports and serotonin transporter gene moderate depression in maltreated children. Proceedings of the National Academy of Sciences of the United States of America 101, 17316e17321.

146

10. Understanding resilience: biological approaches in at-risk populations

Keary, T.A., Hughes, J.W., Palmieri, P.A., 2009. Women with posttraumatic stress disorder have larger decreases in heart rate variability during stress tasks. International Journal of Psychophysiology 73, 257e264. Kessler, R.C., Petukhova, M., Sampson, N.A., Zaslavsky, A.M., Wittchen, H.U., 2012. Twelve-month and lifetime prevalence and lifetime morbid risk of anxiety and mood disorders in the United States. International Journal of Methods in Psychiatric Research 21, 169e184. Kitayama, N., Quinn, S., Bremner, J.D., 2006. Smaller volume of anterior cingulate cortex in abuse-related posttraumatic stress disorder. Journal of Affective Disorders 90, 171e174. Kolassa, I., Kolassa, S., Ertl, V., Papassotiropoulos, A., De Quervain, D.J., 2010. The risk of posttraumatic stress disorder after trauma depends on traumatic load and the catechol-O-methyltransferase Val158Met polymorphism. Biological Psychiatry 67, 304e308. Lawford, B.R., Morris, C.P., Swagell, C.D., Hughes, I.P., Young, R.M., Voisey, J., 2013. NOS1AP is associated with increased severity of PTSD and depression in untreated combat veterans. Journal of Affective Disorders 147, 87e93. Lesch, K., Bengel, D., Heils, A., Sabol, S.A., Greenberg, B.D., Petri, S., et al., 1996. Association of anxiety-related traits with a polymorphism in the serotonin transporter gene regulatory region. Science 274, 1527e1531. Liberzon, I., Abelson, J.L., Flagel, S.B., Raz, J., Young, E.A., 1999. Neuroendocrine and psychophysiologic responses in PTSD: a symptom provocation study. Neuropsychopharmacology 21, 40e50. Liddell, B.J., Kemp, A.H., Steel, Z., Nickerson, A., Bryant, R.A., Tam, N., et al., 2016. Heart rate variability and the relationship between trauma exposure age, and psychopathology in a post-conflict setting. BMC Psychiatry 16, 133. Lindauer, R.J., Vlieger, E.J., Jalink, M., Olff, M., Carlier, I.V., Majoie, C.B., et al., 2004. Smaller hippocampal volume in Dutch police officers with posttraumatic stress disorder. Biological Psychiatry 56, 356e363. Lindauer, R.J., Vlieger, E.J., Jalink, M., Olff, M., Carlier, I.V., Majoie, C.B., et al., 2005. Effects of psychotherapy on hippocampal volume in out-patients with post-traumatic stress disorder: a MRI investigation. Psychological Medicine 35, 1421e1431. Lommen, M.J., Engelhard, I.M., Sijbrandij, M., van den Hout, M.A., Hermans, D., 2013. Pre-trauma individual differences in extinction learning predict posttraumatic stress. Behaviour Research and Therapy 51, 63e67. Minassian, A., Geyer, M.A., Baker, D.G., Nievergelt, C.M., O’Connor, D.T., Risbrough, V.B., MRS Team, 2014. Heart rate variability characteristics in a large group of active-duty marines and relationship to posttraumatic stress. Psychosomatic Medicine 76, 292. Morey, R.A., Gold, A.L., LaBar, K.S., Beall, S.K., Brown, V.M., Haswell, C.C., et al., 2012. Amygdala volume changes in posttraumatic stress disorder in a large case-controlled veterans group. Archives of General Psychiatry 69, 1169e1178. Morgan, C.A., Grillon, C., Lubin, H., Southwick, S.M., 1997. Startle reflex abnormalities in women with sexual assault-related posttraumatic stress disorder. American Journal of Psychiatry 154, 1076e1080. Morgan, C.A., Grillon, C., Southwick, S.M., Davis, M., Charney, D.S., 1995. Fear-potentiated startle in posttraumatic stress disorder. Biological Psychiatry 38, 378e385. Morgan, C.A., Grillon, C., Southwick, S.M., Davis, M., Charney, D.S., 1996. Exaggerated acoustic startle reflex in Gulf War veterans with posttraumatic stress disorder. American Journal of Psychiatry 153, 64e68. Nievergelt, C.M., Maihofer, A.X., Mustapic, M., Yurgil, K.A., Schork, N.J., Miller, M.W., et al., 2015. Genomic predictors of combat stress vulnerability and resilience in US Marines: a genome-wide association study across multiple ancestries implicates PRTFDC1 as a potential PTSD gene. Psychoneuroendocrinology 51, 459e471. Norrholm, S.D., Jovanovic, T., Olin, I.W., Sands, L.A., Karapanou, I., Bradley, B., Ressler, K.J., 2011. Fear extinction in traumatized civilians with posttraumatic stress disorder: relation to symptom severity. Biological Psychiatry 69, 556e563. Norrholm, S.D., Jovanovic, T., Smith, A.K., Binder, E., Klengel, T., Conneely, K., et al., 2013. Differential genetic and epigenetic regulation of catechol-O-methyltransferase is associated with impaired fear inhibition in posttraumatic stress disorder. Frontiers in Behavioral Neuroscience 7, 1e10. O’Doherty, D.C., Chitty, K.M., Saddiqui, S., Bennett, M.R., Lagopoulos, J., 2015. A systematic review and metaanalysis of magnetic resonance imaging measurement of structural volumes in posttraumatic stress disorder. Psychiatry Research 232, 1e33. Ong, A.D., Bergeman, C.S., Boker, S.M., 2009. Resilience comes of age: defining features in later adulthood. Journal of Personality 77, 1777e1804.

References

147

Orr, S.P., Lasko, N.B., Shalev, A.Y., Pitman, R.K., 1995. Physiologic responses to loud tones in Vietnam veterans with posttraumatic stress disorder. Journal of Abnormal Psychology 104, 75. Orr, S.P., Metzger, L.J., Lasko, N.B., Macklin, M.L., Peri, T., Pitman, R.K., 2000. De novo conditioning in traumaexposed individuals with and without posttraumatic stress disorder. Journal of Abnormal Psychology 109, 290. Park, J.E., Lee, J.Y., Kang, S.H., Choi, J.H., Kim, T.Y., So, H.S., Yoon, I.Y., 2017. Heart rate variability of chronic posttraumatic stress disorder in the Korean veterans. Psychiatry Research 255, 72e77.  Pavic, L., Gregurek, R., Rados, M., Brkljacic, B., Brajkovic, L., Simetin-Pavi c, I., et al., 2007. Smaller right hippocampus in war veterans with posttraumatic stress disorder. Psychiatry Research: Neuroimaging 154, 191e198. Pietrzak, R.H., Johnson, D.C., Goldstein, M.B., Malley, J.C., Rivers, A.J., Morgan, C.A., Southwick, S.M., 2010. Psychosocial buffers of traumatic stress, depressive symptoms, and psychosocial difficulties in veterans of Operations Enduring Freedom and Iraqi Freedom: the role of resilience, unit support, and postdeployment social support. Journal of Affective Disorders 120, 188e192. Pitman, R.K., Lanes, D.M., Williston, S.K., Guillaume, J.L., Metzger, L.J., Gehr, G.M., et al., 2001. Psychophysiologic assessment of posttraumatic stress disorder in breast cancer patients. Psychosomatics 42, 133e140. Pitman, R.K., Orr, S.P., Forgue, D.F., Altman, B., de Jong, J.B., Herz, L.R., 1990. Psychophysiologic responses to combat imagery of Vietnam veterans with posttraumatic stress disorder versus other anxiety disorders. Journal of Abnormal Psychology 99, 49. Polanczyk, G., Caspi, A., Williams, B., Price, T.S., Danese, A., Sugden, K., et al., 2009. Protective effect of CRHR1 gene variants on the development of adult depression following childhood maltreatment: replication and extension. Archives of General Psychiatry 66, 978e985. Pole, N., 2007. The psychophysiology of posttraumatic stress disorder: a meta-analysis. Psychological Bulletin 133, 725e746. Pole, N., Neylan, T.C., Otte, C., Henn-Hasse, C., Metzler, T.J., Marmar, C.R., 2009. Prospective prediction of posttraumatic stress disorder symptoms using fear potentiated auditory startle responses. Biological Psychiatry 65, 235e240. Ressler, K.J., Mercer, K.B., Bradley, B., Jovanovic, T., Mahan, A., Kerley, K., et al., 2011. Post-traumatic stress disorder is associated with PACAP and the PAC1 receptor. Nature 470, 492e497. Reynaud, E., Guedj, E., Souville, M., Trousselard, M., Zendjidjian, X., El Khoury-Malhame, M., et al., 2013. Relationship between emotional experience and resilience: an fMRI study in fire-fighters. Neuropsychologia 51, 845e849. Rocha-Rego, V., Pereira, M.G., Oliveira, L., Mendlowicz, M.V., Fiszman, A., Marques-Portella, C., et al., 2012. Decreased premotor cortex volume in victims of urban violence with posttraumatic stress disorder. PLoS One 7, e42560. Rogers, M.A., Yamasue, H., Abe, O., Yamada, H., Ohtani, T., Iwanami, A., et al., 2009. Smaller amygdala volume and reduced anterior cingulate gray matter density associated with history of post-traumatic stress disorder. Psychiatry Research: Neuroimaging 174, 210e216. Rothbaum, B.O., Kozak, M.J., Foa, E.B., Whitaker, D.J., 2001. Posttraumatic stress disorder in rape victims: autonomic habituation to auditory stimuli. Journal of Traumatic Stress 14, 283e293. Rubin, M., Shvil, E., Papini, S., Chhetry, B.T., Helpman, L., Markowitz, J.C., et al., 2016. Greater hippocampal volume is associated with PTSD treatment response. Psychiatry Research 252, 36e39. Sack, M., Hopper, J.W., Lamprecht, F., 2004. Low respiratory sinus arrhythmia and prolonged psychophysiological arousal in posttraumatic stress disorder: heart rate dynamics and individual differences in arousal regulation. Biological Psychiatry 55, 284e290. Sahar, T., Shalev, A.Y., Porges, S.W., 2001. Vagal modulation of responses to mental challenge in posttraumatic stress disorder. Biological Psychiatry 49, 637e643. Sarapas, C., Cai, G., Bierer, L.M., Golier, J.A., Galea, S., Ising, M., et al., 2011. Genetic markers for PTSD risk and resilience among survivors of the world trade center attacks. Disease Markers 30, 101e110. Schinka, J.A., Busch, R.M., Robichaux-Keene, N., 2004. A meta-analysis of the association between the serotonin transporter gene polymorphism (5-HTTLPR) and trait anxiety. Molecular Psychiatry 9, 197e202. Schumacher, S., Schnyder, U., Furrer, M., Mueller-Pfeiffer, C., Wilhelm, F.H., Moergeli, H., et al., 2013. Startle reactivity in the long-term after severe accidental injury: preliminary data. Psychiatry Research 210, 570e574. Shah, A.J., Lampert, R., Goldberg, J., Veledar, E., Bremner, J.D., Vaccarino, V., 2013. Posttraumatic stress disorder and impaired autonomic modulation in male twins. Biological Psychiatry 73, 1103e1110.

148

10. Understanding resilience: biological approaches in at-risk populations

Shalev, A.Y., Freedman, S., Peri, T., Brandes, D., Sahar, T., Orr, S.P., Pitman, R.K., 1998. Prospective study of posttraumatic stress disorder and depression following trauma. American Journal of Psychiatry 155, 630e637. Shalev, A.Y., Peri, T., Gelpin, E., Orr, S.P., Pitman, R.K., 1997. Psychophysiologic assessment of mental imagery of stressful events in Israeli civilian posttraumatic stress disorder patients. Comprehensive Psychiatry 38, 269e273. Sijbrandij, M., Engelhard, I.M., Lommen, M.J., Leer, A., Baas, J.M., 2013. Impaired fear inhibition learning predicts the persistence of symptoms of posttraumatic stress disorder (PTSD). Journal of Psychiatric Research 47, 1991e1997. Smith, B.W., Dalen, J., Wiggins, K., Tooley, E., Christopher, P., Bernard, J., 2008. The brief resilience scale: assessing the ability to bounce back. International Journal of Behavioral Medicine 15, 194e200. Southwick, S.M., Vythilingam, M., Charney, D.S., 2005. The psychobiology of depression and resilience to stress: implications for prevention and treatment. Annual Review of Clinical Psychology 1, 255e291. Stein, M.B., Campbell-Sills, L., Gelernter, J., 2009. Genetic variation in 5HTTLPR is associated with emotional resilience. American Journal of Medical Genetics Part B 150, 900e906. Stein, M.B., Schork, N.J., Gelernter, J., 2008. Gene-by-environment (serotonin transporter and childhood maltreatment) interaction for anxiety sensitivity, an intermediate phenotype for anxiety disorders. Neuropsychopharmacology 33, 312e319. van Rooij, S.J., Stevens, J.S., Ely, T.D., Fani, N., Smith, A.K., Kerley, K.A., et al., 2016. Childhood trauma and COMT genotype interact to increase hippocampal activation in resilient individuals. Frontiers in Psychiatry 7, 156. Villarreal, G., Petropoulos, H., Hamilton, D.A., Rowland, L.M., Horan, W.P., Griego, J.A., et al., 2002. Proton magnetic resonance spectroscopy of the hippocampus and occipital white matter in PTSD: preliminary results. The Canadian Journal of Psychiatry 47, 666e670. Volokhov, R.N., Demaree, H.A., 2010. Spontaneous emotion regulation to positive and negative stimuli. Brain and Cognition 73, 1e6. Wang, Z., Neylan, T.C., Mueller, S.G., Lenoci, M., Truran, D., Marmar, C.R., et al., 2010. Magnetic resonance imaging of hippocampal subfields in posttraumatic stress disorder. Archives of General Psychiatry 67, 296e303. Watson, D., Clark, L.A., Tellegan, A., 1988. Development and validation of brief measures of positive and negative affect: the PANAS scales. Journal of Personality and Social Psychology 54, 1063e1070. Weniger, G., Lange, C., Sachsse, U., Irle, E., 2008. Amygdala and hippocampal volumes and cognition in adult survivors of childhood abuse with dissociative disorders. Acta Psychiatrica Scandinavica 118, 281e290. Wignall, E.L., Dickson, J.M., Vaughan, P., Farrow, T.F., Wilkinson, I.D., Hunter, M.D., et al., 2004. Smaller hippocampal volume in patients with recent-onset posttraumatic stress disorder. Biological Psychiatry 56, 832e836. Windle, G., Bennett, K.M., Noyes, J., 2011. A methodological review of resilience measurement scales. Health and Quality of Life Outcomes 9. Wingo, A.P., Almi, L.M., Stevens, J.J., Klengel, T., Uddin, M., Li, Y., et al., 2015. DICER1 and microRNA regulation in post-traumatic stress disorder with comorbid depression. Nature Communications 6. Wingo, A.P., Almli, L.M., Stevens, J.S., Jovanovic, T., Wingo, T.S., Tharp, G., et al., 2016. Genome-wide association study of positive emotion identifies a genetic variant and a role for microRNAs. Molecular Psychiatry 22, 774e783. Wingo, A.P., Wrenn, G., Pelletier, T., Gutman, A.R., Bradley, B., Ressler, K.J., 2010. Moderating effects of resilience on depression in individuals with a history of childhood abuse or trauma exposure. Journal of Affective Disorders 126, 411e414. Woodward, S.H., Kaloupek, D.G., Streeter, C.C., Martinez, C., Schaer, M., Eliez, S., 2006. Decreased anterior cingulate volume in combat-related PTSD. Biological Psychiatry 59, 582e587. Yehuda, R., Golier, J.A., Tischler, L., Harvey, P.D., Newmark, R., Yang, R.K., Buchsbaum, M.S., 2007. Hippocampal volume in aging combat veterans with and without post-traumatic stress disorder: relation to risk and resilience factors. Journal of Psychiatric Research 41, 435e445. Zalsman, G., Huang, Y.Y., Oquendo, M.A., Burke, A.K., Hu, X.Z., Brent, D.A., et al., 2006. Association of a triallelic serotonin transporter gene promoter region (5-HTTLPR) polymorphism with stressful life events and severity of depression. American Journal of Psychiatry 163, 1588e1593. Zhang, J., Tan,, Q., Yin,, H., Zhang,, X., Huan,, Y., Tang,, L., et al., 2011. Decreased gray matter volume in the left hippocampus and bilateral calcarine cortex in coal mine flood disaster survivors with recent onset PTSD. Psychiatry Research: Neuroimaging 192, 84e90.

C H A P T E R

11

Stress resilience as a consequence of early-life adversity 1

Jakob Hartmann1, Mathias V. Schmidt2 McLean Hospital e Harvard Medical School, Mailman Research Center, Neurobiology of Fear Laboratory, Belmont, MA, United States; 2Max Planck Institute of Psychiatry, Munich, Germany

Introduction Living in Western societies seems to become ever more unpredictable and uncontrollable, and therefore people are being exposed to substantial stressors throughout their lives. In parallel, there is a growing awareness that stress exposure can be harmful and promote a variety of diseases (de Kloet et al., 2005). As a consequence, stress has been labeled the “bad guy,” and the response to stress is often seen as maladaptive. However, it should be considered that humans evolved in a highly unpredictable and uncontrollable environment and that life during the millennia of human evolution was unlikely to be less stressful than it is today. It is therefore not surprising that even today most people are highly resilient in the face of adversity and, despite continuous exposure to a multitude of stressors, stay healthy until old age. But what differentiates individuals that suffer from stress exposure from those that are resilient? In this chapter, we will discuss evidence that suggests the answer to this question may lie in the developmental history and the interplay of experiences early and later in life.

Early-life stressddefinition of the term Before discussing the consequences of early adversity, one has to clearly define what exactly is meant by “early” and how “adversity” or “stress” is defined. For the purpose of the chapter, the early-life period will be considered to cover the prenatal and postnatal period until adolescence, both in animals and in humans. It is clear that the developmental trajectory of mice and humans is quite different depending on the physiological system or brain circuit of interest (Rice and Barone, 2000; Semple et al., 2013). However, overall, it is well accepted that during Stress Resilience https://doi.org/10.1016/B978-0-12-813983-7.00011-2

149

Copyright © 2020 Elsevier Inc. All rights reserved.

150

11. Stress resilience as a consequence of early-life adversity

phases of intense brain development, the organism is most sensitive to environmental stimuli that can lastingly affect the individual (Chen and Baram, 2016; Yam et al., 2015). The term “stress,” on the other hand, is here defined in the sense of unpredictable and uncontrollable adversity, rather than a challenge to body’s homeostasis per se. Here, we are following the arguments laid out by Koolhaas and colleagues, arguing that especially the unpredictable and uncontrollable nature of a challenge is of the essence when interpreting stress in the context of disease or disease risk (Koolhaas et al., 2011).

Early-life stress is a risk factor for psychiatric disorders Stress and traumatic events are well-established risk factors for various pathologies including cardiovascular disease, obesity, diabetes, alcohol and drug abuse, and most prominently psychiatric diseases such as mood and anxiety disorders. Many epidemiological studies strongly support an association of adverse life events particularly during childhood, with increased susceptibility to develop psychopathology later in life (Lupien et al., 2009; Nemeroff, 2016; Kim and Cicchetti, 2006; Larkin and Read, 2008; Weber et al., 2008). More precisely, early-life stress, defined as neglect, physical abuse, sexual abuse, and emotional maltreatment, has been associated with an increased prevalence of depressive disorders such as major depressive disorder (MDD) (Goldberg, 1994; Chapman et al., 2004; Kaufman, 1991; Widom et al., 2007; Scott et al., 2010; Benjet et al., 2010; Maniglio, 2010; Teicher et al., 2006; Anda et al., 2002; Green et al., 2010; Danese et al., 2009), bipolar disorder (Anda et al., 2007; Gilman et al., 2015; Daruy-Filho et al., 2011; Agnew-Blais and Danese, 2016), and with increased rates of anxiety disorders including general anxiety disorder and panic disorder (Copeland et al., 2013; Scott et al., 2010; Green et al., 2010; Cougle et al., 2010). In addition, a history of childhood maltreatment has been linked to subsequent development of posttraumatic stress disorder (PTSD) (Duncan et al., 1996; Widom, 1999; Cabrera et al., 2007; Fritch et al., 2010; Scott et al., 2010) and to an increased risk for suicide attempt (Felitti et al., 1998; Dube et al., 2001; Maniglio, 2011). Notably, psychiatric disorders tend to emerge earlier in previously maltreated individuals, with greater severity, higher comorbidity rates, and with a less favorable response to treatment (Nanni et al., 2012; Alvarez et al., 2011; Leverich et al., 2002; Widom et al., 2007; Benjet et al., 2010). All of this suggests that childhood adversity can lay a fragile foundation for health across the life span.

Early-life stress shapes adult phenotypes Although the exact molecular mechanisms underlying the effects of early-life stress are not fully understood, there is strong evidence for an impairment of the hypothalamic-pituitaryadrenal (HPA) axis (Lupien et al., 2009). In response to stressful stimuli, circulating glucocorticoids (GCs), which are the main hormonal endpoint of the HPA axis, act on numerous organ tissues to execute a wide range of functions involving the immune, digestive, and endocrine systems and including the regulation of the negative feedback of the stress response mostly at the level of the hypothalamus and pituitary (de Kloet et al., 2005). GCs also affect the morphology and

Early-life stress shapes adult phenotypes

151

functionality of central nervous system target tissues, including those responsible for mood and cognitive functions relevant to psychiatric disorders (Abercrombie et al., 2011; de Quervain et al., 2003; Karst et al., 2002; Mitra and Sapolsky, 2010; Hartmann et al., 2017; Gershon, Sudheimer, Tirouvanziam, Williams and O’Hara, 2013; Barik et al., 2013; Tronche et al., 1999; Gass et al., 2001; Anacker et al., 2011). In fact, various types of early-life stress can alter HPA responsiveness and regulation. For instance, HPA axis hyperactivity has been documented in depressed individuals as a consequence of childhood adversity associated with social, physiological, and pharmacological stressors (Heim et al., 2000, 2002; Heim et al., 2001; Heim et al., 2008; Carvalho Fernando et al., 2012). In contrast, other studies point to blunted HPA axis activity, both in healthy subjects and in individuals with mood disorders or PTSD following early-life adversity (Ouellet-Morin et al., 2011; Carpenter et al., 2009; Carpenter et al., 2011; King et al., 2001). Thus, the direction and pattern of such HPA alterations may depend not only on various factors including, but not limited to, the nature and timing of the stressor, and severity and number of traumatic events, but also on genetic and epigenetic markers. It is indisputable that not all individuals who are exposed to early-life adversity will develop psychiatric pathology later in life. So what makes some individuals more susceptible to early-life adversity than others? It is well established that stress-related psychiatric disorders such as PTSD and MDD often originate from gene-environment interactions. Besides genetic variants (e.g., risk alleles), epigenetic modifications, such as histone modifications and DNA (de-)methylation, are thought to mediate long-lasting effects of adverse life events on gene regulation by shaping the transcriptional activity of genes without changing the underlying genetic code (Klengel and Binder, 2015; Pena et al., 2014). These molecular changes induce long-lasting alterations in gene expression and ultimately behavior. Thus, specific genetic and epigenetic markers may moderate the effects of early adversity on later psychopathology. By studying a single nucleotide polymorphism (SNP) within the gene encoding the FK506 binding protein 51 (FKBP51), Klengel et al. (2013) present compelling evidence for an epigenetic mechanism mediating gene  environment interactions that ultimately results in an increased risk to develop stress-related psychiatric disorders. FKBP51 participates in inhibition of glucocorticoid receptor (GR) activity, which is the main mediator of the HPA axis. At the same time, GR activation is involved in the induction of FKBP51 transcription, creating an intracellular, ultrashort feedback loop that regulates GR sensitivity (Wochnik et al., 2005; Touma et al., 2011; Hartmann et al., 2012; Binder, 2009). Klengel et al. elegantly showed that a functional SNP altering chromatin interaction between the transcription start site and longrange enhancers in the FKBP51 gene increases the risk of developing stress-related psychiatric disorders in adulthood through allele-specific, childhood trauma-dependent DNA demethylation in functional glucocorticoid response elements of FKBP51. This demethylation was associated with increased stress-dependent gene transcription followed by a long-term dysregulation of the HPA axis (Klengel et al., 2013). Along these lines, there is a large body of literature linking other genetic risk factors with increased vulnerability to early-life stress, including SNPs in 5-HTT, PACAP, PAC1, Nr3c1, and Crhr1 (Tyrka et al., 2016; Bradley et al., 2008; Caspi et al., 2003; Ressler et al., 2011). Interestingly, Arloth et al. (2015) showed that different sets of genetic variants may be involved in baseline transcriptional regulation versus regulation following environmental impact, such as trauma exposure.

152

11. Stress resilience as a consequence of early-life adversity

In addition to HPA axis dysregulation and specific genetic/epigenetic risk factors, there is increasing evidence for persistent structural and functional consequences of adverse early-life events on specific brain structures and circuits. Indeed, MRI-based studies revealed decreased connectivity between the amygdala and insula/hippocampus, amygdala, and ventromedial prefrontal cortex (vmPFC), as well as between the hippocampus and vmPFC in individuals with a history of childhood maltreatment (van der Werff et al., 2013; Burghy et al., 2012; Herringa et al., 2013). Structural variations in the prefrontal cortex have also been shown to mediate the relationship between early childhood stress and spatial working memory (Hanson et al., 2012). Moreover, previous work has repeatedly shown that hippocampal atrophy in depressed patients is associated with a history of early-life stress (Vythilingam et al., 2002; Buss et al., 2007; Frodl et al., 2010). This raises the question whether psychiatric disorders that are linked to early-life stress occur in response to epigenetic and/or gene expression changes in specific brain circuits. Despite the presented examples, the effects of early-life stress on psychopathology in adulthood remain a challenging process to study in humans. Some of the major difficulties are the duration of such studies, the genetic diversity of humans as well as biases in the perception and reporting of early adversity. Preclinical studies using rodent models have been helpful and crucial in improving our basic knowledge about the consequences of adverse events during development on psychopathology later in life. Such models can offer better control over genetic backgrounds (e.g., inbred strains), onset, type and intensity of stressors and provide much shorter study times. Nevertheless, it is important to note that stress-related psychiatric disorders represent very complex diseases, making it impossible to model certain symptoms in rodents, for example, features of MDD (e.g., feelings of guilt, low self-esteem, or suicidality). However, other core aspects such as anhedonia, loss of motivation, sleep disturbances, anxiety, cognitive deficits, and a dysregulation of the HPA axis do have equivalents in rodents. Thus, it is possible to reach a certain level of face validity with such animal models. Various rodent early-life stress paradigms have been developed in the past. Maternal separation, models based on naturally occurring differences in maternal care, impoverished postnatal environment as well as pharmacological approaches rank among most frequently used. Numerous studies with rats have linked early maternal separation with lifelong neuroendocrine alterations, changes in HPA axis activity, and anxiety-related behavior (Ladd et al., 1996; Rots et al., 1996; Sutanto et al., 1996; Workel et al., 2001; Workel et al., 1997; Suchecki et al., 2000; Kalinichev et al., 2002; Benekareddy et al., 2011). Another model that is based on individual differences of maternal care was first introduced by the group of Michael Meaney (Liu et al., 1997). In this paradigm, abnormal maternal care is defined as increased or decreased (one standard deviation above or below the group mean) licking, grooming, and/or arched-back nursing during the first 10 days of life. Liu et al. (1997) demonstrated that as adults, the offspring of mothers that exhibited more licking and grooming of pups (LG mothers) during the postnatal observation period showed reduced HPA axis responses to acute stress, increased hippocampal GR mRNA expression, and decreased levels of hypothalamic CRF mRNA. Strikingly, each measure was correlated with the frequency of maternal licking and grooming (Liu et al., 1997). In addition, Caldji et al. (1998) reported that individual differences in the frequency of maternal care during infancy regulate the

Early-life stress shapes adult phenotypes

153

development of neural systems mediating the expression of fearfulness in the rat. Interestingly, this paradigm also suggests that maternal effects may modulate optimal cognitive functioning in environments varying in demand in later life, with offspring of high and low LG mothers showing enhanced learning under contexts of low and high stress, respectively (Champagne et al., 2008). The limited nesting material paradigm was first introduced by the group of Tallie Baram. In this model, the mothers (rats and mice) are provided with a limited quantity of bedding and nesting material during postnatal day 2e9 (Ivy et al., 2008; Rice et al., 2008). This manipulation induces perturbed dam-pup interactions reflected in frequent changes of maternal behavior and inconsistent or fragmented maternal care, resulting in a higher stress exposure of the offspring. This adverse environment leads to elevated basal HPA axis activity, increased anxiety-related behavior, and impaired learning and memory functions in adult mice and rats (Rice et al., 2008; Ivy et al., 2008, 2010; Wang et al., 2012; Brunson et al., 2005). Other studies focus on the postnatal application of GCs such as the synthetic GC dexamethasone (Dex) to mimic an early-life stress exposure. Neonatal Dex treatment leads to altered HPA axis activity in response to stress in adolescent and adult rats (Flagel et al., 2002; Neal et al., 2004). Moreover, rats with early-life Dex exposure also show increased anxiety- and depression-like behavior later in life (Neal et al., 2004; Ko et al., 2014; Felszeghy et al., 1993). Again, many psychiatric disorders are caused by interactions between a (epi-) genetic predisposition and environmental factors. Thus, studies assessing the joint contribution of genetic and environmental factors in the etiology of mental disorders have become increasingly important in the field of psychiatry. Rodent models of early-life stress are often combined with additional genetic or environmental (risk) factors, for example, a specific genetic knockout or a second trauma/stress exposure later in life. Applying such experimental designs, corticotropin-releasing factor (CRF) and its receptor, CRFR1 have been shown to modulate the negative effects of early-life stress on cognition and structural plasticity in mice (Wang et al., 2011). Along these lines, we were able to demonstrate that the cell adhesion molecule nectin-3 is required for the effects of CRFR1 on cognition and structural remodeling after early-life stress exposure (Wang et al., 2013). In another study, the limited nesting material paradigm led to impaired social recognition and increased aggression in adult mice, accompanied by increased expression levels of the neuronal cell adhesion molecule neuroligin-2 in the hippocampus. Although hippocampal overexpression of neuroligin-2 in adult mice mimicked the early-life stresseinduced alterations, knockdown of neuroligin-2 in adulthood attenuated the early-life stresseinduced behavioral changes (Kohl et al., 2015). Recently, Peña and colleagues reported that mice subjected to a model of maternal separation were less resilient to chronic defeat in adulthood. In this study, they demonstrated that genes regulated by the transcription factor orthodenticle homeobox 2 (Otx2) in the ventral tegmental area (VTA) primed the response toward susceptibility in adulthood. Although transient juvenile knockdown of Otx2 in VTA mimics the effects of early adversity by increasing stress vulnerability, its overexpression reduces the effects of early-life stress (Pena et al., 2017). Although alterations in HPA axis activity as well as (epi-)genetic risk factors have been implicated in mediating the effects of early-life stress on later psychopathology, there is increasing belief that these changes may occur in a circuit-specific manner. Such circuitlevel framework has been extensively investigated in adult stress models. For example, the

154

11. Stress resilience as a consequence of early-life adversity

combination of optogenetic technology and chronic social defeat stress has identified important structural and functional alterations within the brain’s reward circuits that are associated with aspects of depression and addiction (reviewed in Russo and Nestler (2013); Han and Nestler, (2017); Sparta et. al., (2013)). However, studies focusing on circuit-specific changes in animal models of early-life stress are still largely lacking. Consequently, the utilization of novel technologies including optogenetics, chemogenetics, and diverse viral tools will be instrumental in identifying distinct neuronal populations and brain networks that are affected by early-life stress. Above we highlighted a number of pioneering human and preclinical studies on the complex subject of early-life adversity and its contribution to psychopathology in adulthood. Additional excellent examples are reviewed in Krugers et al. (2017); Nemeroff (2016); Schmidt (2010); Yam et al. (2015); Sandi and Haller (2015); Teicher et al. (2016). All of these studies underscore the ability of early-life stress to exert drastic effects on neurodevelopment and consequently impact health in adulthood. This occurs through interaction with genetic factors and/or reprogramming of the epigenome, which can induce changes in gene expression, cellular and synaptic function, circuit connectivity, and ultimately behavior.

What is the rationale for shaping adult phenotypes by early-life experiences? Although it is clear that adversity during early life has a lasting impact on an individual, the underlying rationale behind such a costly programming is often neglected. As the involved mechanisms of early-life programming have evolved over millennia and are conserved across species, it is likely that they serve an adaptive purpose. In this context, Ellis and Del Giudice proposed the adaptive calibration model (ACM) as an evolutionarydevelopmental theory of individual differences in stress responsivity (Del Giudice, Ellis and Shirtcliff, 2011; Ellis and Del Giudice, 2014). At its core, the ACM states that exposure to stress does not so much impair development per se but directs or regulates it toward strategies that are adaptive under similarly stressful conditions. As a consequence, the ACM suggests that under high-stress conditions animals adapt to a “fast lifestyle,” where, for example, heightened aggression (males) or enhanced vigilance increases individual fitness. Along the same lines, the ecologists Sheriff and Love have argued that the consequences of early-life stress exposure should be viewed under the broad concept of a life history perspective, both for the mother and the offspring (Sheriff and Love, 2013). Phenotypic traits following early-life adversity, for example, heightened anxiety, are not determining individual fitness per se but instead alter fitness upon interaction with the environment. A classic example is the predator-prey population cycle of snowshoe hares, where the maternal environment shapes the number and phenotype of the offspring in a highly adaptive fashion. Thus, the multiple examples of adverse early-life stress effects collected in both humans and animals as reviewed earlier may represent just one side of the coin and predominantly occur when early-life and adult-life environments do not match. This concept is especially relevant for psychiatric disorders, arguing that a mismatch of early-life and adult-life stress exposure might be the crucial risk factor for depression, rather than stress exposure per se (Schmidt, 2011). As the consequences of environmental stress exposure are directly related to the

Evidence for the match/mismatch theory in animal studies

155

genetic susceptibility of an individual (Belsky et al., 2009; Belsky, 2016), one would expect that only individuals with a high (epi)genetic flexibility (environmental sensitivity) would thrive under matched environmental conditions, whereas for individuals with a lower environmental sensitivity, stress effects may be rather cumulative over time (Nederhof and Schmidt, 2012). It is important to point out, however, that there is a difference between the evolved, adaptive function of the stress response and apparent pathological consequences for the individual (Flinn et al., 2011). Thus, natural selection that favors evolutionary (reproductive) fitness may not maximize short-term physical and mental health. With these considerations in mind, the question arises of whether there is evidence from human or preclinical studies to support the match/mismatch hypothesis.

Evidence for the match/mismatch theory in humans Although few studies directly addressed the possibility that exposure to adversity early in life may enhance stress resilience in adulthood, there are still a number of examples where such an effect was indeed observed. Especially, exposure to moderate levels of early-life stress protects children from an overshooting HPA axis in response to the Trier Social Stress Test (TSST), compared with both severe and no early-life stress exposure (Gunnar et al., 2009). A very nice example for the beneficial effects of adversity even in healthy controls is the study from Seery and colleagues, who show that individuals with some lifetime adversity report better mental health and well-being outcomes than people with high or no lifetime adversity (Seery et al., 2010). Accordingly, healthy volunteers with a moderate history of lifetime adversity displayed less negative responses to pain and more positive psychophysiological responses compared with individuals with no or high history of lifetime adversity (Seery et al., 2013). Along the same lines, a history of moderate childhood adversity was shown to be associated with an enhanced capacity of emotional regulation that was also reflected in an active suppression of the activity in limbic brain circuits (Schweizer et al., 2016). Shapero et al. (2015) demonstrated that adolescents with a history of moderate stressful life events are significantly protected from depressiogenic effects of proximal stressful events (Shapero et al., 2015). In contrast, especially, individuals with mismatched childhood and recent stress levels have been shown to display abnormal resting state functional connectivity in pathways responsible for social and cognitive functioning (Paquola et al., 2017). Stress exposure can also still have adaptive consequences when it occurs later during development, especially during adolescence. For example, adults who are exposed to work stress as adolescents have been shown to exhibit fewer negative psychiatric health side effects from work-related stress as adults (Mortimer and Staff, 2004). Furthermore, adolescents with a history of early adversity were protected from a stress-induced increase in depression vulnerability as adults, if their attention style was categorized as highly flexible (Nederhof, 2013). Thus, people with a genetic background favoring fast and flexible adaptation benefit from matching environments early and late in life, even if both are stressful. Taken together, these examples demonstrate that early exposure to especially moderate levels of adversity may indeed facilitate the development of stress resilience, especially in individuals that are highly susceptible to environmental influences.

156

11. Stress resilience as a consequence of early-life adversity

Evidence for the match/mismatch theory in animal studies Although most studies in humans suffer from retrospective assessment of early-life adversity and high variability of environmental conditions, animal studies can be designed much more stringently and allow the exact control of environmental conditions. Unfortunately, most studies addressing a possible interaction of early-life and adult-life environments have not utilized a continuous exposure to moderate adversity but rather single exposures of mostly severe stressors over a short developmental period, with the majority of the developmental time consisting of standard housing without adversity. The consequent results are then often interpreted in the light of cumulative stress exposure or the two-hit hypothesis (Pena et al., 2017). An alternative explanation would be that these study designs enhance the mismatch situation of environmental conditions, as programming during the nonstress periods, which also takes place during potentially essential developmental phases, suggested a safe and stress-free environment. Nonetheless, there are still numerous examples in the preclinical literature that support the mismatch hypothesis and argue for an adaptive role of early adversity by enhancing stress resilience. Already the early and seminal work of Levine and colleagues suggested that moderate levels of stress exposure early in life may have beneficial effects when animals were exposed to threats in adulthood (Levine, 1959, 1962). Later on, Dienstbier (1989) proposed the concept of psychophysiological toughness, where defined and controllable adversity early in life fosters subsequent stress resilience. Even environmental enrichment during early life and adolescence can be viewed as form of chronic mild stress, resulting in stress inoculation and consequently resilience to stress exposure in adulthood (Crofton et al., 2015). Also more recently, these concepts could be confirmed and extended. For example, female Balb/C mice have been shown to be less affected by social isolation in adulthood when exposed to erratic maternal care because of limited nesting and bedding material following birth (Santarelli et al., 2014). Interestingly, these results were dependent on the estrous cycle phase the animals were tested in. Furthermore, early-life stress in the form of limited nesting and bedding material dampened the behavioral and endocrine response to chronic social stress in adolescence (Hsiao et al., 2016). Similarly, male mice exposed to the same earlylife stress paradigm displayed a heightened resilience to chronic social defeat stress in adulthood (Santarelli et al., 2017). Brockhurst et al. (2015) reported that experience of early chronic social stress reduced HPA axis activity and improved stress-coping and anxiety-like behavior following subsequent repeated restraint. Along these lines, prenatally stressed offspring are more likely to become subordinate in a social group as adulthood but are more resilient to that social status compared with offspring from nonstressed mothers (Scott et al., 2017). Beneficial effects seem also evident in combination with treatment, as, for example, lymphocytes from chronically stressed mice confer antidepressant-like effects to stress-naïve mice (Brachman et al., 2015), arguing that especially the immune system benefits from some form of stress inoculation early in life. There even seem to be transgenerational effects of early-life stress to promote a proresilient phenotype (Gapp et al., 2014). Stress exposure early in life also seems to be beneficial for cognitive performance, especially under challenging conditions. In rats, chronic unpredictable stress during adolescence was shown to improve foraging-related problem solving under high-threat conditions

Conclusions

157

(Chaby et al., 2015). Along these lines, repeated social stress improves performance in an attentional set-shifting task (Chaijale et al., 2015). Furthermore, exposure to moderate peripubertal stress, where the effects of chronic mild stress were buffered by social partners in the home cage, was shown to result in an improved cognitive phenotype in aged female mice (Morrison et al., 2016). The molecular underpinnings that are responsible for the beneficial effects of moderate stress exposure for later stress resilience are far from understanding, and only a few studies have addressed this question. For example, Biggio et al. (2014) showed that maternal deprivation dampens corticosterone response to adult social isolation in rats, which was paralleled by not only hippocampal brain-derived neurotrophic factor (BDNF) expression. BDNF but also the depression risk factor SLC6A15 (Hyde et al., 2016; Kohli et al., 2011), which were also implicated as potential mechanism in the study by Santarelli et al. (2014) in female mice. Moreover, in another study, prenatal stress induced anxiety and HPA axis hyper-reactivity, which could be normalized by exposure to adolescent chronic mild stress, together with a normalization of hippocampal tryptophan hydroxylase 2 expression (Van den Hove et al., 2013). However, all of these findings are just correlational observations, and so far there is a clear lack of mechanistic studies that would shed some light on the causal relationships that lead to enhanced stress resilience following adversity during development.

Conclusions Taken together, there seems to be compelling evidence from human and animal studies that supports the match/mismatch hypothesis, while at the same time there are of course numerous reports that rather argue for a cumulative stress effect. Part of this apparent discrepancy can be explained by the different study designs, and as so often the devil is in the details. Most importantly, especially for animal studies, one has to consider the severity of the stressors and their chronicity. To trigger an adaptive long-term response, stressors likely need to be in the mild to moderate range and be present over developmentally meaningful time frames. If stress exposures are short (but severe) and interlaced with long periods of stress-free environments, successful adaptation is less likely. Finally, the genetic heritage of an individual will be decisive in their adaptive capacity to stress, so that some individuals will be more prone to suffer under chronic stress conditions, whereas others adapt and thrive (Fig. 11.1). It will be challenging to unravel the molecular and cellular mechanisms that determine this complex gene  early environment  adult environment interaction. One caveat in most of the current stress-related research is the limitation to only a few species and selected genetic backgrounds, which by itself limits the diversity and possible outcomes of stress exposure. However, these challenges can be overcome, and in the end a better understanding of the risks as well as the benefits of stress exposure will also advance our understanding of stress-related disorders and improve options of successful intervention.

158

11. Stress resilience as a consequence of early-life adversity

FIGURE 11.1

Schematic representation of two extremes in the reaction to early-life adversity. The black line represents a condition where the individual has a genetic background that is insensitive to environmental experiences and therefore prone to adverse cumulative effects of stress exposure. Consequently, such an individual would thrive under mild stress conditions and become more and more vulnerable to stress with higher levels of developmental stress exposure. The blue line represents the other extreme, namely an individual that has a high sensitivity to the environment. Such an individual would benefit from moderate stress levels during development and become stress resilient. In contrast, with no meaningful stress exposure during development, such an individual will have no chance to adapt its physiology to an adverse environment and will therefore be highly vulnerable to stress in adulthood. Severe early-life stress exposure is expected to be harmful under most circumstances, largely independent of the genetic background of an individual.

References Abercrombie, H.C., Jahn, A.L., Davidson, R.J., Kern, S., Kirschbaum, C., Halverson, J., 2011. Cortisol’s effects on hippocampal activation in depressed patients are related to alterations in memory formation. Journal of Psychiatric Research 45, 15e23. Agnew-Blais, J., Danese, A., 2016. Childhood maltreatment and unfavourable clinical outcomes in bipolar disorder: a systematic review and meta-analysis. Lancet Psychiatry 3, 342e349. Alvarez, M.J., Roura, P., Osés, A., Foguet, Q., Sola, J., Arrufat, F.X., 2011. Prevalence and clinical impact of childhood trauma in patients with severe mental disorders. The Journal of Nervous and Mental Disease 199, 156e161. Anacker, C., Zunszain, P.A., Carvalho, L.A., Pariante, C.M., 2011. The glucocorticoid receptor: pivot of depression and of antidepressant treatment? Psychoneuroendocrinology 36, 415e425. Anda, R.F., Brown, D.W., Felitti, V.J., Bremner, J.D., Dube, S.R., Giles, W.H., 2007. Adverse childhood experiences and prescribed psychotropic medications in adults. American Journal of Preventive Medicine 32, 389e394. Anda, R.F., Whitfield, C.L., Felitti, V.J., Chapman, D., Edwards, V.J., Dube, S.R., et al., 2002. Adverse childhood experiences, alcoholic parents, and later risk of alcoholism and depression. Psychiatric Services 53, 1001e1009.

References

159

Arloth, J., Bogdan, R., Weber, P., Frishman, G., Menke, A., Wagner, K.V., et al., 2015. Genetic differences in the immediate transcriptome response to stress predict risk-related brain function and psychiatric disorders. Neuron 86, 1189e1202. Barik, J., Marti, F., Morel, C., Fernandez, S.P., Lanteri, C., Godeheu, G., et al., 2013. Chronic stress triggers social aversion via glucocorticoid receptor in dopaminoceptive neurons. Science 339, 332e335. Belsky, J., Jonassaint, C., Pluess, M., Stanton, M., Brummett, B., Williams, R., 2009. Vulnerability genes or plasticity genes? Molecular Psychiatry 14, 746e754. Belsky, J., 2016. The differential susceptibility hypothesis: sensitivity to the environment for better and for worse. JAMA Pediatrics 170, 321e322. Benekareddy, M., Vadodaria, K.C., Nair, A.R., Vaidya, V.A., 2011. Postnatal serotonin type 2 receptor blockade prevents the emergence of anxiety behavior, dysregulated stress-induced immediate early gene responses, and specific transcriptional changes that arise following early life stress. Biological Psychiatry 70, 1024e1032. Benjet, C., Borges, G., Medina-Mora, M. a. E., 2010. Chronic childhood adversity and onset of psychopathology during three life stages: childhood, adolescence and adulthood. Journal of Psychiatric Research 44, 732e740. Biggio, F., Pisu, M.G., Garau, A., Boero, G., Locci, V., Mostallino, M.C., et al., 2014. Maternal separation attenuates the effect of adolescent social isolation on HPA axis responsiveness in adult rats. European Neuropsychopharmacology 24, 1152e1161. Binder, E.B., 2009. The role of FKBP5, a co-chaperone of the glucocorticoid receptor in the pathogenesis and therapy of affective and anxiety disorders. Psychoneuroendocrinology 34 (Suppl. 1), S186eS195. Brachman, R.A., Lehmann, M.L., Maric, D., Herkenham, M., 2015. Lymphocytes from chronically stressed mice confer antidepressant-like effects to naive mice. Journal of Neuroscience 35, 1530e1538. Bradley, R.G., Binder, E.B., Epstein, M.P., Tang, Y., Nair, H.P., Liu, W., et al., 2008. Influence of child abuse on adult depression: moderation by the corticotropin-releasing hormone receptor gene. Archives of General Psychiatry 65, 190e200. Brockhurst, J., Cheleuitte-Nieves, C., Buckmaster, C.L., Schatzberg, A.F., Lyons, D.M., 2015. Stress inoculation modeled in mice. Translational Psychiatry 5, e537. Brunson, K.L., Kramar, E., Lin, B., Chen, Y., Colgin, L.L., Yanagihara, T.K., et al., 2005. Mechanisms of late-onset cognitive decline after early-life stress. Journal of Neuroscience 25, 9328e9338. Burghy, C.A., Stodola, D.E., Ruttle, P.L., Molloy, E.K., Armstrong, J.M., Oler, J.A., et al., 2012. Developmental pathways to amygdala-prefrontal function and internalizing symptoms in adolescence. Nature Neuroscience 15, 1736e1741. Buss, C., Lord, C., Wadiwalla, M., Hellhammer, D.H., Lupien, S.J., Meaney, M.J., et al., 2007. Maternal care modulates the relationship between prenatal risk and hippocampal volume in women but not in men. Journal of Neuroscience 27, 2592e2595. Cabrera, O.A., Hoge, C.W., Bliese, P.D., Castro, C.A., Messer, S.C., 2007. Childhood adversity and combat as predictors of depression and post-traumatic stress in deployed troops. American Journal of Preventive Medicine 33, 77e82. Caldji, C., Tannenbaum, B., Sharma, S., Francis, D., Plotsky, P.M., Meaney, M.J., 1998. Maternal care during infancy regulates the development of neural systems mediating the expression of fearfulness in the rat. Proceedings of the National Academy of Sciences of the United States of America 95, 5335e5340. Carpenter, L.L., Shattuck, T.T., Tyrka, A.R., Geracioti, T.D., Price, L.H., 2011. Effect of childhood physical abuse on cortisol stress response. Psychopharmacology 214, 367e375. Carpenter, L.L., Tyrka, A.R., Ross, N.S., Khoury, L., Anderson, G.M., Price, L.H., 2009. Effect of childhood emotional abuse and age on cortisol responsivity in adulthood. Biological Psychiatry 66, 69e75. Carvalho Fernando, S., Beblo, T., Schlosser, N., Terfehr, K., Otte, C., Löwe, B., et al., 2012. Associations of childhood trauma with hypothalamic-pituitary-adrenal function in borderline personality disorder and major depression. Psychoneuroendocrinology 37, 1659e1668. Caspi, A., Sugden, K., Moffitt, T.E., Taylor, A., Craig, I.W., Harrington, H., et al., 2003. Influence of life stress on depression: moderation by a polymorphism in the 5-HTT gene. Science 301, 386e389. Chaby, L.E., Sheriff, M.J., Hirrlinger, A.M., Braithwaite, V.A., 2015. Does early stress prepare individuals for a stressful future? Stress during adolescence improves foraging under threat. Animal Behaviour 105, 37e45. Chaijale, N.N., Snyder, K., Arner, J., Curtis, A.L., Valentino, R.J., 2015. Repeated social stress increases reward salience and impairs encoding of prediction by rat locus coeruleus neurons. Neuropsychopharmacology 40, 513e523.

160

11. Stress resilience as a consequence of early-life adversity

Champagne, D.L., Bagot, R.C., van Hasselt, F., Ramakers, G., Meaney, M.J., de Kloet, E.R., et al., 2008. Maternal care and hippocampal plasticity: evidence for experience-dependent structural plasticity, altered synaptic functioning, and differential responsiveness to glucocorticoids and stress. Journal of Neuroscience 28, 6037e6045. Chapman, D.P., Whitfield, C.L., Felitti, V.J., Dube, S.R., Edwards, V.J., Anda, R.F., 2004. Adverse childhood experiences and the risk of depressive disorders in adulthood. Journal of Affective Disorders 82, 217e225. Chen, Y., Baram, T.Z., 2016. Toward understanding how early-life stress reprograms cognitive and emotional brain networks. Neuropsychopharmacology 41, 197e206. Copeland, W.E., Wolke, D., Angold, A., Costello, E.J., 2013. Adult psychiatric outcomes of bullying and being bullied by peers in childhood and adolescence. JAMA Psychiatry 70, 419e426. Cougle, J.R., Timpano, K.R., Sachs-Ericsson, N., Keough, M.E., Riccardi, C.J., 2010. Examining the unique relationships between anxiety disorders and childhood physical and sexual abuse in the National Comorbidity Survey-Replication. Psychiatry Research 177, 150e155. Crofton, E.J., Zhang, Y., Green, T.A., 2015. Inoculation stress hypothesis of environmental enrichment. Neuroscience & Biobehavioral Reviews 49, 19e31. Danese, A., Moffitt, T.E., Harrington, H., Milne, B.J., Polanczyk, G., Pariante, C.M., et al., 2009. Adverse childhood experiences and adult risk factors for age-related disease: depression, inflammation, and clustering of metabolic risk markers. Archives of Pediatrics and Adolescent Medicine 163, 1135e1143. Daruy-Filho, L., Brietzke, E., Lafer, B., Grassi-Oliveira, R., 2011. Childhood maltreatment and clinical outcomes of bipolar disorder. Acta Psychiatrica Scandinavica 124, 427e434. de Kloet, E.R., Joëls, M., Holsboer, F., 2005. Stress and the brain: from adaptation to disease. Nature Reviews Neuroscience 6, 463e475. de Quervain, D.J.F., Henke, K., Aerni, A., Treyer, V., McGaugh, J.L., Berthold, T., et al., 2003. Glucocorticoid-induced impairment of declarative memory retrieval is associated with reduced blood flow in the medial temporal lobe. European Journal of Neuroscience 17, 1296e1302. Del Giudice, M., Ellis, B.J., Shirtcliff, E.A., 2011. The adaptive calibration model of stress responsivity. Neuroscience & Biobehavioral Reviews 35, 1562e1592. Dienstbier, R.A., 1989. Arousal and physiological toughness: implications for mental and physical health. Psychological Review 96, 84e100. Dube, S.R., Anda, R.F., Felitti, V.J., Chapman, D.P., Williamson, D.F., Giles, W.H., 2001. Childhood abuse, household dysfunction, and the risk of attempted suicide throughout the life span: findings from the Adverse Childhood Experiences Study. Journal of the American Medical Association 286, 3089e3096. Duncan, R.D., Saunders, B.E., Kilpatrick, D.G., Hanson, R.F., Resnick, H.S., 1996. Childhood physical assault as a risk factor for PTSD, depression, and substance abuse: findings from a national survey. American Journal of Orthopsychiatry 66, 437e448. Ellis, B.J., Del Giudice, M., 2014. Beyond allostatic load: rethinking the role of stress in regulating human development. Development and Psychopathology 26, 1e20. Felitti, V.J., Anda, R.F., Nordenberg, D., Williamson, D.F., Spitz, A.M., Edwards, V., et al., 1998. Relationship of childhood abuse and household dysfunction to many of the leading causes of death in adults. The Adverse Childhood Experiences (ACE) Study. American Journal of Preventive Medicine 14, 245e258. Felszeghy, K., Sasvari, M., Nyakas, C., 1993. Behavioral depression: opposite effects of neonatal dexamethasone and ACTH-(4-9) analogue (ORG 2766) treatments in the rat. Hormones and Behavior 27, 380e396. Flagel, S.B., Vazquez, D.M., Watson, S.J., Neal, C.R., 2002. Effects of tapering neonatal dexamethasone on rat growth, neurodevelopment, and stress response. American Journal of Physiology e Regulatory, Integrative and Comparative Physiology 282, R55eR63. Flinn, M.V., Nepomnaschy, P.A., Muehlenbein, M.P., Ponzi, D., 2011. Evolutionary functions of early social modulation of hypothalamic-pituitary-adrenal axis development in humans. Neuroscience & Biobehavioral Reviews 35, 1611e1629. Fritch, A.M., Mishkind, M., Reger, M.A., Gahm, G.A., 2010. The impact of childhood abuse and combat-related trauma on postdeployment adjustment. Journal of Traumatic Stress 23, 248e254. Frodl, T., Reinhold, E., Koutsouleris, N., Reiser, M., Meisenzahl, E.M., 2010. Interaction of childhood stress with hippocampus and prefrontal cortex volume reduction in major depression. Journal of Psychiatric Research 44, 799e807. Gapp, K., Soldado-Magraner, S., Alvarez-Sanchez, M., Bohacek, J., Vernaz, G., Shu, H., et al., 2014. Early life stress in fathers improves behavioural flexibility in their offspring. Nature Communications 5.

References

161

Gass, P., Reichardt, H.M., Strekalova, T., Henn, F., Tronche, F., 2001. Mice with targeted mutations of glucocorticoid and mineralocorticoid receptors: models for depression and anxiety? Physiology & Behavior 73, 811e825. Gershon, A., Sudheimer, K., Tirouvanziam, R., Williams, L.M., O’Hara, R., 2013. The long-term impact of early adversity on late-life psychiatric disorders. Current Psychiatry Reports 15, 352. Gilman, S.E., Ni, M.Y., Dunn, E.C., Breslau, J., McLaughlin, K.A., Smoller, J.W., et al., 2015. Contributions of the social environment to first-onset and recurrent mania. Molecular Psychiatry 20, 329e336. Goldberg, R.T., 1994. Childhood abuse, depression, and chronic pain. The Clinical Journal of Pain 10, 277e281. Green, J.G., McLaughlin, K.A., Berglund, P.A., Gruber, M.J., Sampson, N.A., Zaslavsky, A.M., et al., 2010. Childhood adversities and adult psychiatric disorders in the national comorbidity survey replication I: associations with first onset of DSM-IV disorders. Archives of General Psychiatry 67, 113e123. Gunnar, M.R., Frenn, K., Wewerka, S.S., Van Ryzin, M.J., 2009. Moderate versus severe early life stress: associations with stress reactivity and regulation in 10-12-year-old children. Psychoneuroendocrinology 34, 62e75. Han, M.H., Nestler, E.J., 2017. Neural substrates of depression and resilience. Neurotherapeutics 14, 677e686. Hanson, J.L., Chung, M.K., Avants, B.B., Rudolph, K.D., Shirtcliff, E.A., Gee, J.C., et al., 2012. Structural variations in prefrontal cortex mediate the relationship between early childhood stress and spatial working memory. Journal of Neuroscience 32, 7917e7925. Hartmann, J., Dedic, N., Pöhlmann, M.L., Häusl, A., Karst, H., Engelhardt, C., et al., 2017. Forebrain glutamatergic, but not GABAergic, neurons mediate anxiogenic effects of the glucocorticoid receptor. Molecular Psychiatry 22, 466e475. Hartmann, J., Wagner, K.V., Liebl, C., Scharf, S.H., Wang, X.D., Wolf, M., et al., 2012. The involvement of FK506binding protein 51 (FKBP5) in the behavioral and neuroendocrine effects of chronic social defeat stress. Neuropharmacology 62, 332e339. Heim, C., Newport, D.J., Bonsall, R., Miller, A.H., Nemeroff, C.B., 2001. Altered pituitary-adrenal axis responses to provocative challenge tests in adult survivors of childhood abuse. American Journal of Psychiatry 158, 575e581. Heim, C., Newport, D.J., Heit, S., Graham, Y.P., Wilcox, M., Bonsall, R., et al., 2000. Pituitary-adrenal and autonomic responses to stress in women after sexual and physical abuse in childhood. Journal of the American Medical Association 284, 592e597. Heim, C., Newport, D.J., Mletzko, T., Miller, A.H., Nemeroff, C.B., 2008. The link between childhood trauma and depression: insights from HPA axis studies in humans. Psychoneuroendocrinology 33, 693e710. Heim, C., Newport, D.J., Wagner, D., Wilcox, M.M., Miller, A.H., Nemeroff, C.B., 2002. The role of early adverse experience and adulthood stress in the prediction of neuroendocrine stress reactivity in women: a multiple regression analysis. Depression and Anxiety 15, 117e125. Herringa, R.J., Birn, R.M., Ruttle, P.L., Burghy, C.A., Stodola, D.E., Davidson, R.J., et al., 2013. Childhood maltreatment is associated with altered fear circuitry and increased internalizing symptoms by late adolescence. Proceedings of the National Academy of Sciences of the United States of America 110, 19119e19124. Hsiao, Y.M., Tsai, T.C., Lin, Y.T., Chen, C.C., Huang, C.C., Hsu, K.S., 2016. Early life stress dampens stress responsiveness in adolescence: evaluation of neuroendocrine reactivity and coping behavior. Psychoneuroendocrinology 67, 86e99. Hyde, C.L., Nagle, M.W., Tian, C., Chen, X., Paciga, S.A., Wendland, J.R., et al., 2016. Identification of 15 genetic loci associated with risk of major depression in individuals of European descent. Nature Genetics 48, 1031e1036. Ivy, A.S., Brunson, K.L., Sandman, C., Baram, T.Z., 2008. Dysfunctional nurturing behavior in rat dams with limited access to nesting material: a clinically relevant model for early-life stress. Neuroscience 154, 1132e1142. Ivy, A.S., Rex, C.S., Chen, Y., Dube, C., Maras, P.M., Grigoriadis, D.E., et al., 2010. Hippocampal dysfunction and cognitive impairments provoked by chronic early-life stress involve excessive activation of CRH receptors. Journal of Neuroscience 30, 13005e13015. Kalinichev, M., Easterling, K.W., Plotsky, P.M., Holtzman, S.G., 2002. Long-lasting changes in stress-induced corticosterone response and anxiety-like behaviors as a consequence of neonatal maternal separation in LongâV“Evans rats. Pharmacology Biochemistry and Behavior 73, 131e140. Karst, H., Nair, S., Velzing, E., Rumpff-van Essen, L., Slagter, E., Shinnick-Gallagher, P., et al., 2002. Glucocorticoids alter calcium conductances and calcium channel subunit expression in basolateral amygdala neurons. European Journal of Neuroscience 16, 1083e1089. Kaufman, J., 1991. Depressive disorders in maltreated children. Journal of the American Academy of Child & Adolescent Psychiatry 30, 257e265.

162

11. Stress resilience as a consequence of early-life adversity

Kim, J., Cicchetti, D., 2006. Longitudinal trajectories of self-system processes and depressive symptoms among maltreated and nonmaltreated children. Child Development 77, 624e639. King, J.A., Mandansky, D., King, S., Fletcher, K.E., Brewer, J., 2001. Early sexual abuse and low cortisol. Psychiatry Clin.Neurosci. 55, 71e74. Klengel, T., Binder, E.B., 2015. Epigenetics of stress-related psychiatric disorders and gene x environment interactions. Neuron 86, 1343e1357. Klengel, T., Mehta, D., Anacker, C., Rex-Haffner, M., Pruessner, J.C., Pariante, C.M., et al., 2013. Allele-specific FKBP5 DNA demethylation mediates gene-childhood trauma interactions. Nature Neuroscience 16, 33e41. Ko, M.C., Hung, Y.H., Ho, P.Y., Yang, Y.L., Lu, K.T., 2014. Neonatal glucocorticoid treatment increased depressionlike behaviour in adult rats. The International Journal of Neuropsychopharmacology 17, 1995e2004. Kohl, C., Wang, X.D., Grosse, J., Fournier, C., Harbich, D., Westerholz, S., et al., 2015. Hippocampal neuroligin-2 links early-life stress with impaired social recognition and increased aggression in adult mice. Psychoneuroendocrinology 55, 128e143. Kohli, M.A., Lucae, S., Saemann, P.G., Schmidt, M.V., Demirkan, A., Hek, K., et al., 2011. The neuronal transporter gene SLC6A15 confers risk to major depression. Neuron 70, 252e265. Koolhaas, J.M., Bartolomucci, A., Buwalda, B., De Boer, S.F., Flügge, G., Korte, S.M., et al., 2011. Stress revisited: a critical evaluation of the stress concept. Neuroscience & Biobehavioral Reviews 35, 1291e1301. Krugers, H.J., Arp, J.M., Xiong, H., Kanatsou, S., Lesuis, S.L., Korosi, A., et al., 2017. Early life adversity: lasting consequences for emotional learning. Neurobiol Stress 6, 14e21. Ladd, C.O., Owens, M.J., Nemeroff, C.B., 1996. Persistent changes in corticotropin-releasing factor neuronal systems induced by maternal deprivation. Endocrinology 137, 1212e1218. Larkin, W., Read, J., 2008. Childhood trauma and psychosis: evidence, pathways, and implications. Journal of Postgraduate Medicine 54, 287e293. Leverich, G.S., McElroy, S.L., Suppes, T., Keck, P.E., Denicoff, K.D., Nolen, W.A., et al., 2002. Early physical and sexual abuse associated with an adverse course of bipolar illness. Biological Psychiatry 51, 288e297. Levine, S., 1959. Emotionality and aggressive behavior in the mouse as a function of infantile experience. The Journal of Genetic Psychology 94, 77e83. Levine, S., 1962. Plasma-free corticosteroid response to electric shock in rats stimulated in infancy. Science 135, 795e796. Liu, D., Diorio, J., Tannenbaum, B., Caldji, C., Francis, D., Freedman, A., et al., 1997. Maternal care, hippocampal glucocorticoid receptors. And Hypothalamic-Pituitary-Adrenal Responses to Stress 277, 1659e1662. Lupien, S.J., McEwen, B.S., Gunnar, M.R., Heim, C., 2009. Effects of stress throughout the lifespan on the brain, behaviour and cognition. Nature Reviews Neuroscience 10, 434e445. Maniglio, R., 2011. The role of child sexual abuse in the etiology of suicide and non-suicidal self-injury. Acta Psychiatrica Scandinavica 124, 30e41. Maniglio, R., 2010. Child sexual abuse in the etiology of depression: a systematic review of reviews. Depression and Anxiety 27, 631e642. Mitra, R., Sapolsky, R.M., 2010. Gene therapy in rodent amygdala against fear disorders. Expert Opinion on Biological Therapy 10, 1289e1303. Morrison, K.E., Narasimhan, S., Fein, E., Bale, T.L., 2016. Peripubertal stress with social support promotes resilience in the face of aging. Endocrinology 157, 2002e2014. Mortimer, J.T., Staff, J., 2004. Early work as a source of developmental discontinuity during the transition to adulthood. Development and Psychopathology 16, 1047e1070. Nanni, V., Uher, R., Danese, A., 2012. Childhood maltreatment predicts unfavorable course of illness and treatment outcome in depression: a meta-analysis. American Journal of Psychiatry 169, 141e151. Neal, C.R., Weidemann, G., Kabbaj, M., Vázquez, D.M., 2004. Effect of neonatal dexamethasone exposure on growth and neurological development in the adult rat. American Journal of Physiology - Regulatory, Integrative and Comparative Physiology 287, R375eR385. Nederhof, E., 2013. Developmental mismatch: why some immigrants seem protected from affective, personality, and substance use disorders. JAMA Psychiatry 70, 1374e1375. Nederhof, E., Schmidt, M.V., 2012. Mismatch or cumulative stress: toward an integrated hypothesis of programming effects. Physiology & Behavior 106, 691e700.

References

163

Nemeroff, C.B., 2016. Paradise lost: the neurobiological and clinical consequences of child abuse and neglect. Neuron 89, 892e909. Ouellet-Morin, I., Odgers, C.L., Danese, A., Bowes, L., Shakoor, S., Papadopoulos, A.S., et al., 2011. Blunted cortisol responses to stress signal social and behavioral problems among maltreated/bullied 12-year-old children. Biological Psychiatry 70, 1016e1023. Paquola, C., Bennett, M.R., Hatton, S.N., Hermens, D.F., Lagopoulos, J., 2017. Utility of the cumulative stress and mismatch hypotheses in understanding the neurobiological impacts of childhood abuse and recent stress in youth with emerging mental disorder. Human Brain Mapping 38, 2709e2721 (n/a). Pena, C.J., Bagot, R.C., Labonté, B., Nestler, E.J., 2014. Epigenetic signaling in psychiatric disorders. Journal of Molecular Biology 426, 3389e3412. Pena, C.J., Kronman, H.G., Walker, D.M., Cates, H.M., Bagot, R.C., Purushothaman, I., et al., 2017. Early life stress confers lifelong stress susceptibility in mice via ventral tegmental area OTX2. Science 356, 1185e1188. Ressler, K.J., Mercer, K.B., Bradley, B., Jovanovic, T., Mahan, A., Kerley, K., et al., 2011. Post-traumatic stress disorder is associated with PACAP and the PAC1 receptor. Nature 470, 492e497. Rice, C., Sandman, C.A., Lenjavi, M.R., Baram, T.Z., 2008. A novel mouse model for acute and long-lasting consequences of early life stress. Endocrinology 149, 4892e4900. Rice, D., Barone, S., 2000. Critical periods of vulnerability for the developing nervous system: evidence from humans and animal models. Environmental Health Perspectives 108 (Suppl. 3), 511e533. Rots, N.Y., de Jong, J., Workel, J.O., Levine, S., Cools, A.R., De Kloet, E.R., 1996. Neonatal maternally deprived rats have as adults elevated basal pituitary-adrenal activity and enhanced susceptibility to apomorphine. Journal of Neuroendocrinology 8, 501e506. Russo, S.J., Nestler, E.J., 2013. The brain reward circuitry in mood disorders. Nature Reviews Neuroscience 14, 609e625. Sandi, C., Haller, J., 2015. Stress and the social brain: behavioural effects and neurobiological mechanisms. Nature Reviews Neuroscience 16, 290e304. Santarelli, S., Lesuis, S.L., Wang, X.D., Wagner, K.V., Hartmann, J., Labermaier, C., et al., 2014. Evidence supporting the match/mismatch hypothesis of psychiatric disorders. European Neuropsychopharmacology 24, 907e918. Santarelli, S., Zimmermann, C., Kalideris, G., Lesuis, S.L., Arloth, J., Uribe, A. s., et al., 2017. An adverse early life environment can enhance stress resilience in adulthood. Psychoneuroendocrinology 78, 213e221. Schmidt, M.V., 2010. Molecular mechanisms of early life stress–lessons from mouse models. Neuroscience & Biobehavioral Reviews 34, 845e852. Schmidt, M.V., 2011. Animal models for depression and the mismatch hypothesis of disease. Psychoneuroendocrinology 36, 330e338. Schweizer, S., Walsh, N.D., Stretton, J., Dunn, V.J., Goodyer, I.M., Dalgleish, T., 2016. Enhanced emotion regulation capacity and its neural substrates in those exposed to moderate childhood adversity. Social Cognitive and Affective Neuroscience 11, 272e281. Scott, K.A., de Kloet, A.D., Smeltzer, M.D., Krause, E.G., Flak, J.N., Melhorn, S.J., et al., 2017. Susceptibility or resilience? Prenatal stress predisposes male rats to social subordination, but facilitates adaptation to subordinate status. Physiology & Behavior 178, 117e125. Scott, K.M., Smith, D.R., Ellis, P.M., 2010. Prospectively ascertained child maltreatment and its association with DSM-IV mental disorders in young adults. Archives of General Psychiatry 67, 712e719. Seery, M.D., Alison, E., Silver, R.C., 2010. Whatever does not kill us: cumulative lifetime adversity, vulnerability, and resilience. Journal of Personality and Social Psychology 99, 1025e1041. Seery, M.D., Leo, R.J., Lupien, S.P., Kondrak, C.L., Almonte, J.L., 2013. An upside to adversity? Moderate cumulative lifetime adversity is associated with resilient responses in the face of controlled stressors. Psychological Science 24, 1181e1189. Semple, B.D., Blomgren, K., Gimlin, K., Ferriero, D.M., Noble-Haeusslein, L.J., 2013. Brain development in rodents and humans: identifying benchmarks of maturation and vulnerability to injury across species. Progress in Neurobiology 106e107, 1e16. Shapero, B.G., Hamilton, J.L., Stange, J.P., Liu, R.T., Abramson, L., Alloy, L.B., 2015. Moderate childhood stress buffers against depressive response to proximal stressors: a multi-wave prospective study of early adolescents. Journal of Abnormal Child Psychology 43, 1403e1413. Sheriff, M.J., Love, O.P., 2013. Determining the adaptive potential of maternal stress. Ecology Letters 16, 271e280.

164

11. Stress resilience as a consequence of early-life adversity

Sparta, D.R., Jennings, J.H., Ung, R.L., Stuber, G.D., 2013. Optogenetic strategies to investigate neural circuitry engaged by stress. Behavioural Brain Research 255, 19e25. Suchecki, D., Duarte Palma, B., Tufik, S., 2000. Pituitary-adrenal axis and behavioural responses of maternally deprived juvenile rats to the open field. Behavioural Brain Research 111, 99e106. Sutanto, W., Rosenfeld, P., De Kloet, E.R., Levine, S., 1996. Long-term effects of neonatal maternal deprivation and ACTH on hippocampal mineralocorticoid and glucocorticoid receptors. Brain Research Developmental Brain Research 92, 156e163. Teicher, M.H., Samson, J.A., Anderson, C.M., Ohashi, K., 2016. The effects of childhood maltreatment on brain structure, function and connectivity. Nature Reviews Neuroscience 17, 652e666. Teicher, M.H., Samson, J.A., Polcari, A., McGreenery, C.E., 2006. Sticks, stones, and hurtful words: relative effects of various forms of childhood maltreatment. American Journal of Psychiatry 163, 993e1000. Touma, C., Gassen, N.C., Herrmann, L., Cheung-Flynn, J., Büll, D.R., Ionescu, I.A., et al., 2011. FK506 binding protein 5 shapes stress responsiveness: modulation of neuroendocrine reactivity and coping behavior. Biological Psychiatry 70, 928e936. Tronche, F., Kellendonk, C., Kretz, O., Gass, P., Anlag, K., Orban, P.C., et al., 1999. Disruption of the glucocorticoid receptor gene in the nervous system results in reduced anxiety. Nature Genetics 23, 99e103. Tyrka, A.R., Ridout, K.K., Parade, S.H., 2016. Childhood adversity and epigenetic regulation of glucocorticoid signaling genes: associations in children and adults. Development and Psychopathology 28, 1319e1331. Van den Hove, D.L.A., Leibold, N.K., Strackx, E., Martinez-Claros, M., Lesch, K.P., Steinbusch, H.W.M., et al., 2013. Prenatal Stress and Subsequent Exposure to Chronic Mild Stress in Rats; Interdependent Effects on Emotional Behavior and the Serotonergic System. van der Werff, S.J.A., Pannekoek, J.N., Veer, I.M., van Tol, M.J., Aleman, A., Veltman, D.J., et al., 2013. Resting-state functional connectivity in adults with childhood emotional maltreatment. Psychological Medicine 43, 1825e1836. Vythilingam, M., Heim, C., Newport, J., Miller, A.H., Anderson, E., Bronen, R., et al., 2002. Childhood trauma associated with smaller hippocampal volume in women with major depression. American Journal of Psychiatry 159, 2072e2080. Wang, X.D., Labermaier, C., Holsboer, F., Wurst, W., Deussing, J.M., Müller, M.B., et al., 2012. Early-life stressinduced anxiety-related behavior in adult mice partially requires forebrain corticotropin-releasing hormone receptor 1. European Journal of Neuroscience 36 (3), 2360e2367, 9-21-2017.Ref Type: Statute. Wang, X.D., Rammes, G., Kraev, I., Wolf, M., Liebl, C., Scharf, S.H., et al., 2011. Forebrain CRF1 modulates early-life stress-programmed cognitive deficits. Journal of Neuroscience 31, 13625e13634. Wang, X.D., Su, Y.A., Wagner, K.V., Avrabos, C., Scharf, S.H., Hartmann, J., et al., 2013. Nectin-3 links CRHR1 signaling to stress-induced memory deficits and spine loss. Nature Neuroscience 16, 706e713. Weber, K., Rockstroh, B., Borgelt, J., Awiszus, B., Popov, T., Hoffmann, K., et al., 2008. Stress load during childhood affects psychopathology in psychiatric patients. BMC Psychiatry 8, 63. Widom, C.S., 1999. Posttraumatic stress disorder in abused and neglected children grown up. American Journal of Psychiatry 156, 1223e1229. Widom, C.S., DuMont, K., Czaja, S.J., 2007. A prospective investigation of major depressive disorder and comorbidity in abused and neglected children grown up. Archives of General Psychiatry 64, 49e56. Wochnik, G.M., Rüegg, J., Abel, G.A., Schmidt, U., Holsboer, F., Rein, T., 2005. FK506-binding proteins 51 and 52 differentially regulate dynein interaction and nuclear translocation of the glucocorticoid receptor in mammalian cells. Journal of Biological Chemistry 280, 4609e4616. Workel, J.O., Oitzl, M.S., Fluttert, M., Lesscher, H., Karssen, A., De Kloet, E.R., 2001. Differential and age-dependent effects of maternal deprivation on the hypothalamic-pituitary-adrenal axis of brown Norway rats from youth to senescence. Journal of Neuroendocrinology 13, 569e580. Workel, J.O., Oitzl, M.S., Ledeboer, A., De Kloet, E.R., 1997. The Brown Norway rat displays enhanced stress-induced ACTH reactivity at day 18 after 24-h maternal deprivation at day 3. Brain Research Developmental Brain Research 103, 199e203. Yam, K.Y., Naninck, E.F.G., Schmidt, M.V., Lucassen, P.J., Korosi, A., 2015. Early-life adversity programs emotional functions and the neuroendocrine stress system: the contribution of nutrition, metabolic hormones and epigenetic mechanisms. Stress 18, 328e342.

C H A P T E R

12

Mechanisms by which early-life experiences promote enduring stress resilience or vulnerability 1

Annabel K. Short1, 2, Jessica L. Bolton1, 2, Tallie Z. Baram1, 2

Department of Anatomy/Neurobiology, University of California-Irvine, Irvine, CA, United States; 2Department of Pediatrics, University of California-Irvine, Irvine, CA, United States

Introduction A predisposition to emotional and cognitive disorders originates early in life (Kessler et al., 2005; Insel, 2009). The concepts of gene-environment interaction, and the importance of earlylife experience for resilience or vulnerability to mental illness, have been demonstrated in both preclinical rodent studies and clinical studies in human populations (Insel, 2009; Bale et al., 2010; Gunnar, 2010; Fox et al., 2010; Juul et al., 2011; Bale, 2015). Resilience is defined as an active and adaptive biological, psychological, and social response to an event that may otherwise impair one’s normal function (Dudley et al. 2011; Russo et al., 2012). Vulnerability is the susceptibility of an individual to a disorder and is often related (in addition to genetics) to early experiences. Resilience or vulnerability to a stressor tends to be regulated by molecular, cellular, synaptic, and finally, behavioral changes that determine the level of coping and normal function. Early-life experiences that contribute to resilience or vulnerability may consist of stimuli from the general environment (poverty, wealth, war). Notably, in view of the crucial importance of interaction/attachment with the primary caretaker for survival (Bowlby, 1950), there is compelling evidence to suggest that sensory signals from the primary caretaker during the neonatal period are vital in determining an individual’s vulnerability or resilience to cognitive and emotional disorders later in life (Meaney and Szyf, 2005; Fenoglio et al., 2006; Lupien et al., 2009; Korosi, 2009; Fox et al., 2010; van Hasselt et al., 2012; Wang et al., 2014). Thus, early-life adversity/stress, as well as beneficial early-life experiences, may be “filtered” by the mother and conveyed to the infant via altered maternal signals.

Stress Resilience https://doi.org/10.1016/B978-0-12-813983-7.00012-4

165

Copyright © 2020 Elsevier Inc. All rights reserved.

166

12. Mechanisms by which early-life experiences promote enduring stress resilience or vulnerability

There is now a large body of evidence in humans associating early-life adversity with emotional and cognitive disorders later in life. These publications range from epidemiological studies of famine or war (Brown et al., 1995; Eriksson et al., 2014) to prospective, cross-sectional, and case-control analyses (e.g., Bremner et al., 1993; Kaplan et al., 2001). To elucidate both direct causal and mechanistic relationships between early-life experiences and outcomes later in life, rodent models have been utilized. A variety of prenatal and postnatal manipulations have been employed in these studies (Molet et al., 2014; Walker et al., 2017). Studies inducing early-life stress are associated with negative emotional consequences, including behaviors that typically signify depression, anxiety, and social isolation. The consequences of early-life (prenatal as well as postnatal) stress on emotional and social behaviors have been a subject of several recent reviews (Lucassen et al., 2013; van Bodegom et al., 2017; Walker et al., 2017). Although not as widely studied, manipulations that result in a more positive early-life environment are associated with increased learning and memory, and decreased anxiety-like phenotypes (Weaver et al., 2004; Fenoglio et al., 2006; Champagne, 2008; Korosi, 2009; Korosi et al., 2010). The type and severity of early-life perturbations determine their consequences. In humans, this effect is highlighted in studies of institutionally raised children. These studies show chronic impoverished care was associated with cognitive and emotional problems. However, the associated consequences were somewhat reversed by fostering into more positive environments, thus highlighting the importance of early interaction with a primary caregiver (Fenoglio et al. 2005, 2006; Gunnar, 2010; Chen et al., 2012; Wang et al., 2014). Abnormal patterns of maternal care, ranging from unpredictable neglect to inconsistency and lack of sensitivity, can be a major cause of early-life stress (Fenoglio et al., 2005; Bota and Swanson, 2007). This is in contrast to repeated, predictable barrages of maternal care. To study early-life experiences, animal models have aimed to recapitulate these conditions by manipulating maternal interactions with the developing individual.

The degree of predictability of maternal care influences long-lasting cognitive and emotional resilience or vulnerability Experimental model studies, in conjunction with human studies, have found that maternal input is the most significant environmental experience during development (Bowlby, 1950; Baram et al., 2012; Kundakovic and Champagne, 2015). Thus, most animal models of early-life stress have manipulated maternal interaction, disrupting either the quantity or quality of maternal care early in life (see Molet et al., 2014; Walker et al., 2017; van Bodegom et al., 2017 for recent reviews).

Studying early-life experiences experimentally Disrupted maternal care Some of the earliest and most informative translational work on early-life stress associated with disrupted maternal care has been performed in nonhuman primates. These models have the advantage of modeling the development of complex psychiatric disorders. Initial nonhuman primate studies were the first to demonstrate the association of intact

The degree of predictability of maternal care influences long-lasting cognitive and emotional resilience or vulnerability

167

maternal-infant interactions with appropriate development of cognitive and emotional phenotypes (Mason and Harlow, 1958). In addition to these studies of disrupted maternal care, a model of physical separation in nonhuman primates has been used. With this approach, Sanchez and colleagues associated adverse early-life experience with altered development of the stress response. This abnormal development resulted in emotional reactivity and manifested as poor maternal care when these infants reached adulthood and became mothers themselves (Maestripieri et al., 2006; Drury et al., 2017). Due to the difficulties associated with nonhuman primate work, most studies of disrupted maternal care are performed in rodents. Although it is difficult to measure sophisticated cognitive and emotional disorders in rodents, appropriate testing and analyses yield tractable results when studying the developmental and behavioral outcomes of early-life stress. Given the similarities in the role of maternal care across species, and the significant parallelism of brain development especially the development of synaptic connectivity and brain circuits, rodents are a suitable model for studying the effects of maternal careerelated stress on neuropsychiatric outcome. Comparable with the maternal role in humans, the rodent dam is the primary source for nutrition and pup well-being. This includes providing protection and safety in the nest, which involves the communication of vital environmental signals from the dam to the pups (Levine, 1957; Eghbal-Ahmadi et al., 1999; Lucassen et al., 2013). Although removing the mother from the pup will effectively disrupt these signals, doing so for extended periods of time will lead to obvious physical stressors such as hypothermia and starvation. To overcome this, studies of disrupted maternal care may employ intermittent maternal separation, for variable lengths of time. This decreases the quantity of time available for maternal care in addition to causing a repeated stress (Millstein and Holmes, 2007). These approaches have been widely employed in the field and have provided an understanding of how directly decreasing maternal care influences early development and outcomes later in life. Yet, these approaches have yielded variable results (Shalev and Kafkafi, 2002; Aisa et al., 2007; Hill et al., 2014). Furthermore, adverse conditions that are commonly experienced by human infants and children include situations such as severe poverty, famine, war, maternal drug abuse, where the child is with the mother and receiving maternal signals. Because of the overwhelming importance of maternal signals, including their nature and patterns, there is a rationale to study early-life adversity in the presence of the mother. To recapitulate poverty in the presence of the mother, a now prevalent approach uses manipulations of the home cage while both the dam and pups are present. During postnatal days (P)2e9, nesting and bedding materials are limited (LBN) (Gilles et al., 1996; Molet et al., 2014; Naninck et al., 2015), and this manipulation reliably and reproducibly causes fragmented and unpredictable maternal behaviors toward the pups (Molet et al., 2016a). This is likely because the impoverished environment induces stress in the dams (Ivy et al., 2008). Notably, the duration or quality of the nurturing behaviors of the dams is minimally altered: it is the patterns of maternal care that are disrupted (Ivy et al., 2008; Rice et al., 2008; Molet et al., 2016a; Walker et al., 2017). This fragmented maternal care causes chronic, unpredictable, and uncontrollable “emotional stress” in the pups (Gilles et al., 1996; Ivy et al., 2008; Rice et al., 2008; Moriceau et al., 2009; Wang et al., 2011; Molet et al., 2014; Naninck et al., 2015). The pups’ stress is apparent in persistent elevation of plasma corticosterone and adrenal hypertrophy, which is associated with emotional and cognitive vulnerabilities in adulthood (Brunson et al., 2005;

168

12. Mechanisms by which early-life experiences promote enduring stress resilience or vulnerability

Rice et al., 2008; Molet et al., 2016b). These cognitive and emotional outcomes produced by the LBN approach have been reliably reproduced by numerous laboratories and multiple outcome measures (Moriceau et al., 2009; Roth et al., 2009; Dalle Molle et al., 2012; Raineki et al., 2012; Gunn et al., 2013; Malter Cohen et al., 2013; Naninck et al., 2015; Walker et al., 2017). Augmented/predictable maternal care Important biological phenomena run along a spectrum. If unpredictable maternal care provokes enduring vulnerability, then highly predictable patterns of maternal-derived sensory signals to the developing brain should promote cognitive and emotional resilience long-term. Nurturing maternal care is typically quantified by licking and grooming behaviors. The handling paradigm (Levine, 1957; Plotsky and Meaney, 1993; Korosi, 2009) has been extensively used to modulate maternal licking and grooming quantity, as well as patterns. This involves a brief (15 min) daily separation of rat pups from the mother during the first weeks of life. The timing of these bouts of separation is crucial, and brief separations will promote increased, predictable sensory input to the pups upon reunion with their mothers (Liu et al., 1997; Fenoglio et al., 2006; Korosi et al., 2010). The recurrent predictable maternal signals lead to increased resilience to depressive-like behavior (Meaney and Szyf, 2005; SinghTaylor et al., 2017) and improved learning and memory (Fenoglio et al., 2005). Notably, it is not simply the increase in maternal care that drives resilience. A single day of handling or irregular handling is insufficient to promote the molecular and behavioral outcomes (Fenoglio et al., 2006). Recurrent, predictable, repetitive brief separations (typically in the same circadian phase) seem to be required (Fenoglio et al., 2006; Karsten and Baram, 2013).

Cognitive and emotional outcomes of early-life experiences The effects of early-life experiences on resilience or vulnerability in adulthood can be examined in rodents using standardized cognitive and emotional tests that are also translational to humans. Tests of emotional behavior such as the forced-swim test are used to identify depressive-like phenotypes in rodents, as when used in conjunction with routinely prescribed antidepressants, there is a reduction in depressive-like behaviors (Slattery and Cryan, 2012). Measures of anxiety have relied on tests such as the open field and elevated-plus maze. Cognitive tests have typically involved memory and especially hippocampus-dependent spatial memory. Available standardized tests for this function include both the wellcharacterized Morris water maze and the object location memory test. The former involves stress/adversity in itself (forced swimming, cold water), whereas the object location relies on natural curiosity and is devoid of stress, as well as the potential confounding effect of early-life experience on stress-related behavior later in life. Thus, spatial memory tests are best when these considerations are included. An additional important caveat is that the large majority of studies have employed males, and many of the tests have been developed and standardized for males. Here, we note if reported studies and outcome pertain to females. A spectrum of cognitive consequences of early-life experiences Memory impairments have been the common outcome in rodents exposed to chronic early-life adversity. For example, in a rigorous and hippocampus-dependent test of object

The degree of predictability of maternal care influences long-lasting cognitive and emotional resilience or vulnerability

169

location memory, an overt impairment in spatial memory was found as early as adolescence in rats reared for a week in the simulated poverty environment (LBN rats) (Molet et al., 2016b). A less rigorous memory task involving object recognition (OR) found comparable performance in LBN versus control adolescent rats during adolescence. However, an acute-stress “challenge” imposed 24 h prior to the test led to memory problems only in the LBN rats, thus unmasking a latent cognitive vulnerability (Molet et al., 2016b). The memory deficits after chronic early-life stress also progressed over the life span of LBN rats, so that deficits in OR memory emerged by middle age (Molet et al., 2016b). At this age, hippocampus-dependent memory deficits were also present using the Morris water maze task (Brunson et al., 2005; Ivy et al., 2010). These data, obtained in males, are intriguing, because the emergence of memory problems during middle age has been found in men experiencing early-life adversity in well-controlled epidemiological studies (Kaplan et al., 2001). Conversely, rats receiving predictable augmented maternal care had improved hippocampusdependent cognitive function (Tang, 2001; Fenoglio et al., 2005; Lesuis et al., 2016). Together, these studies indicate that either naturally occurring or experimentally recurrent, predictable or enhanced sensory stimulation that pups receive from the dam improves hippocampusdependent learning and memory later in life (Korosi and Baram, 2008). Emotional consequences of early-life experience A variety of emotional problems, based on rodent tasks considered indicative of depression or anxiety, have been reported after early-life stress (McEwen, 2003; Molet et al., 2014; Chen and Baram, 2016). Increased anxiety-like behaviors in the elevated-plus maze test were found later in adulthood (Dalle Molle et al., 2012, but see Molet et al., 2016a), Conversely, predictable barrages of maternal care early in life was related to decreased anxiety-like phenotypes in adult rats (Singh-Taylor et al., 2017). Anhedonia, a reduced capacity to experience pleasure, which commonly heralds depression or schizophrenia in humans (Whitton et al., 2015), has been identified in rodents following perturbations of early-life experiences. Already during adolescence, anhedonia, apparent both as a significant reduction in sucrose preference and a reduction of peer play, was found in late-adolescent LBN rats (Molet et al., 2016a; Bolton et al., 2018). This anhedonia was not accompanied by overt anxiety-like behavior or depressive-like behavior. Adolescent anhedonia after early-life stress has since been confirmed in a separate LBN cohort in a different laboratory, as indicated by decreased consumption of palatable food (M&Ms) (Bolton et al., 2019). Furthermore, LBN rats self-administered lower levels of cocaine, consistent with a reduced hedonic set point (Bolton et al., 2019). These changes were shown to be selective to anhedonia, as early-life adversity did not affect other measures of addiction, such as sensitivity to self-administered cocaine dose; responding for cocaine under extinction conditions; or cocaine- or cue-induced reinstatement of cocaine seeking. Early-life adversity did not induce anxiety-like behavior or augmented locomotor response to acute cocaine. Together, these findings demonstrate enduring effects of early-life adversity on reward/ pleasure-circuit function. In contrast, rats that have been handled in the first week of a life, thus receiving recurrent barrages of maternal care signals, when given a similar task, had an increase in the consumption of palatable food, in the absence of an anxiety-like phenotype (Silveira et al., 2005).

170

12. Mechanisms by which early-life experiences promote enduring stress resilience or vulnerability

Although the majority of emotional consequences of chronic early-life adversity have been negative, there is some evidence for positive outcomes following stressful experiences that are challenging but not overwhelming, so-called “stress inoculation” (Lyons, 2009). For example, Lyons and colleagues have demonstrated that exposure of newly weaned squirrel monkeys to brief intermittent maternal separations decreased subsequent anxiety and stress responsivity. This resilience to later stress did not seem to be maternally mediated or related to changes in maternal care, unlike the rodent models discussed above (Parker et al., 2006). It is possible that some discrepancies reported on the emotional consequences of early-life stress may be due to inadvertent generation of recurrent, predictable bouts of maternal care, which may counteract or reverse the stress effects. For example, a recent powerful paper by Peña et al. (2017) did not find major emotional outcomes after early-life stress in the simulated poverty paradigm. Yet, in aiming to improve the approach, Peña et al. added daily maternal separations, which, upon the subsequent return of the dams to the cages, may have provided predictable, recurrent daily episodes of maternal tactile signals (licking) to the pups (Korosi et al., 2010; Singh-Taylor et al., 2015, 2017). Thus, the potential negative consequences of unpredictable and fragmented sensory signals from the mother in the LBN cages on the development of brain circuits were most likely mitigated by the predictable daily barrages of maternal care when the dams were returned to the cages. These variations highlight the complexities inherent in all of our experimental approaches to the human condition. Importantly, the consequences of early-life experiences are clearly further modulated later in life. In humans, fostering at 2 years or earlier clearly ameliorated the effects of institutionalization (Nelson et al., 2007). In rodents, enrichment (Bredy et al., 2003) or pharmacological manipulations (Ivy et al., 2010) at least partially reversed cognitive deficits promoted by early-life adversity. Understanding the basis of these consequences of early-life stress should enable targeted and logical interventions to improve lifelong outcomes.

Mechanisms by which early-life experiences elicit enduring changes in neuronal, circuit, and behavioral functions How altered early-life experience promotes resilience or vulnerability to emotional and cognitive disorders in adulthood is yet to be fully elucidated. An attractive hypothesis is that, in analogy to the development of the visual and auditory brain circuits, early sensory signals from the mother alter synaptic development and pruning, thus influencing the maturation of brain networks involved in emotional and cognitive processing (Bogdan and Hariri, 2012; Burghy et al., 2012; Maras and Baram, 2012; Karsten and Baram, 2013; Singh-Taylor et al., 2015; Chen and Baram, 2016; Davis et al., 2017). Changes in synaptic connectivity, in turn, have recently been shown to influence epigenetic programs in stress-sensitive neurons (Singh-Taylor et al., 2017).

Stress-sensitive neurons in the hypothalamus are influenced by early-life stress as well as by augmented early-life experience Early-life stress and fragmented maternal care have significant effects on the developing and adult stress response system. Abnormal maternal care in the simulated poverty environment provokes an increased number and function of excitatory synapses to stress-sensitive neurons in the hypothalamus (Gunn et al., 2013). In contrast, recurrent predictable maternal

Mechanisms by which early-life experiences elicit enduring changes in neuronal, circuit, and behavioral functions

171

signals reduce the number of excitatory synapses to corticotropin-releasing factor (CRF)e expressing cells in the paraventricular nucleus (PVN) of the hypothalamus (Korosi et al., 2010). Recent exciting data indicate that the change in synapse number and function is sufficient to turn on massive epigenetic/transcriptomic programs in the PVN CRF cells (SinghTaylor et al., 2017). These changes include lifelong reduction in CRF expression in the PVN. Reduced expression of CRF in the PVN is classically associated with reduced CRF release in response to stress throughout life. Thus, there is now a direct mechanistic connection between early-life experiences, development of circuitry of a key element of the stress system, and enduring epigenetic change in the level of expression and function of a stress hormone. Notably, reduced or increased CRF expression and release influences the levels of circulating glucocorticoids, thus providing multiple pathways by which early-life stress or optimal experience will influence the brain long-term (Liu et al., 1997; Eghbal-Ahmadi et al., 1999; Korosi et al., 2010). Rodents, reared in LBN cages have elevated basal levels of serum corticosterone (Brunson et al., 2005; Rice et al., 2008). These changes are present immediately following the stress and may or may not persist into adulthood. Although there is also adrenal hypertrophy described in pups following LBN, these changes do not persist into adulthood (Gilles et al., 1996; Avishai-Eliner et al., 2001; Brunson et al., 2005; Ivy et al., 2008). Conversely, predictable maternal care is associated with decreased release of serum corticosterone in response to stress (Liu et al., 1997; Eghbal-Ahmadi et al., 1999; Meaney and Szyf, 2005; Singh-Taylor et al., 2017).

Memory consequences of early-life stress and experiencesda hippocampal story There is clear vulnerability or resilience accorded by early-life experience to hippocampusdependent tasks. In rodents, early-life stress causes reduction in dorsal hippocampal volume associated with a reduction in dendritic arborization (Brunson et al., 2005; Ivy et al., 2010; Molet et al., 2016b). This is comparable with observations in humans. For example, children raised in orphanages have reduced hippocampal volume (Hodel et al., 2015). Rodent data allow speculation that reduced hippocampal volume in humans is also a result of a decrease in synaptic growth and branching of neuronal dendrites, contributing to the observed functional deficits (Brunson et al., 2005; Radley et al., 2008; Ivy et al., 2010; Maras and Baram, 2012; Chen and Baram, 2016). In addition to structural changes in the hippocampus of rodents following early-life stress, significant reduction in LTP has been observed, which progresses as the animal ages (Brunson et al., 2005). These structural and functional changes in the hippocampus following early-life stress are also associated with lasting molecular changes (Gilles et al., 1996; Avishai-Eliner et al., 2001; Bath et al., 2016). Both elevated plasma corticosteroids and enhanced CRF gene (Crh) expression in hippocampus (Ivy et al., 2010; Maras and Baram, 2012) might play a role in these hippocampal changes. Glucocorticoids powerfully modulate dendritic and synapse growth in hippocampus (Magarinõs and McEwen, 1995; Alfarez et al., 2009; Jafari et al., 2012; Liston et al., 2013), and chronic increases in CRF, via binding to local CRF receptors (Chen et al., 2013) impair dendritic branching and pruning early in life (Chen et al., 2004; Joëls and Baram, 2009). In contrast to the adverse consequences of early-life stress, augmented maternal care may have beneficial influences on the hippocampus, and these also seem to progress with age. Aged rats that have undergone handling at an early age show less hippocampal cell loss when compared with control animals and maintain better cognitive function (Fenoglio et al., 2005).

172

12. Mechanisms by which early-life experiences promote enduring stress resilience or vulnerability

Early-life experiences affect a number of brain systems Early-life experiences provoke enduring changes in the expression of multiple molecules throughout the brain. This is likely mediated via large-scale transcriptional/epigenetic programs (Singh-Taylor et al., 2017; Peña et al., 2017; Gray et al., 2017). Evidence for altered gene expression and function are found in anxiety-fear circuits, including the central nucleus of the amygdala (ACe), and bed nucleus of stria terminalis (BnST). In these regions, changes in Crh gene expression are an eloquent example of broad transcriptional change. In addition, the changes in Crh expression probably directly contribute to altered functional outcomes in behaviors subserved by the underlying circuits. For example, increased Crh expression has been found in amygdala (Dubé et al., 2015) already during adolescence after early-life stress; in contrast, high levels of predictable maternal care promote reduced Crh expression in the ACe (Fenoglio et al., 2004). Notably, the same experience promotes reduction of glucocorticoid receptor (GR) in ACe. As GR occupancy increases CRF expression in the amygdala (Makino et al., 1994), these findings support a coordinate effect of early-life experience on two mediators of the stress system. A second, intercalated circuit influenced by early-life experience encompasses the mesolimbic reward/pleasure circuit. As mentioned above, dysregulation of these systems has been observed following early-life stress. Mechanistically, social play, a pleasurable task, provoked Fos activation of CRF neurons within the ACe, in contrast to controls (Bolton et al., 2018). These findings suggest aberrant connectivity of pleasure/reward and fear/anxiety circuits. Importantly, knockdown of CRF expression in the ACe was sufficient to completely reverse the observed anhedonia in individual LBN rats, suggesting mechanistic roles for CRF-expressing neurons in the amygdala in the abnormal emotional function induced by early-life stress. Aberrant patterns in Fos activation are apparent in LBN rats also following cocaine. The abnormal activation was found in other reward-related regions, such as the core of the nucleus accumbens (NAc) and the lateral habenula (LHb) (Bolton et al., 2019). This evidence for network disruptions following adverse early-life experiences is supported by highresolution MRI studies. Tractography revealed increased tracts/streamlines connecting the amygdala to the medial prefrontal cortex in LBN rats (Bolton et al., 2018). Together, these results suggest that projections in both pleasure/reward- and anxiety/aversion-related circuits are enduringly altered because of early-life stress, which may have functional implications. Although there is currently limited evidence for a role of augmented maternal care in pleasure and reward-seeking behavioral changes, there are reported changes in related brain regions. Fos mapping studies have suggested that the pathway of neuronal activation by repeated barrages of maternal care travels to the hypothalamic PVN via the ACe and BnST (Fenoglio et al., 2006). These high levels of neuronal activation result in robust and enduring suppression of Crh gene expression in these neurons (Fenoglio et al., 2006; Karsten and Baram, 2013), which further supports a role for the CRF neurons in the amygdala in resilience or vulnerabilities to emotional disorders in adulthood.

How the consequences of early-life experience are encoded long-term: transcriptional and epigenetic mechanisms The critical importance of events taking place during sensitive developmental periods is their influence on developmental trajectories and hence their enduring effects (Russo et al., 2012;

Mechanisms by which early-life experiences elicit enduring changes in neuronal, circuit, and behavioral functions

173

Regev and Baram, 2014; Peña et al., 2017). In the context of early-life stress, this is clearly apparent from the ability of interventions, including pharmacological, to alter the course of these consequences if undertaken directly after the stress epoch (Bredy et al., 2003; Ivy et al., 2010). However, interventions several months later were ineffective in reversing the effects of earlylife stress on hippocampal functions (Ivy et al., unpublished observations). There is also evidence for the crucial importance of the sensitive period in humans. Early-life stress is associated with an increased risk of dementia and cognitive problems in middle age (Kaplan et al., 2001; Nelson et al., 2007). Interventions were found only to be effective prior to the first 3 years of life, suggesting that mechanisms behind these behavioral changes decrease in plasticity over time (Nelson et al., 2007; Regev and Baram, 2014). There is increasing evidence that the larger changes in brain circuit behavior induced by early-life stress may occur through molecular changes via epigenetic mechanisms. Commonly described epigenetic mechanisms include DNA methylation, histone modifications, and chromatin remodeling. Alterations to gene expression can occur via noncoding RNAs, which is often also regarded as an epigenetic mechanism. Multiple studies in rodents have shown that aberrant maternal care, whether biological or fostered, will produce permanent changes in behavior and gene expression patterns (Roth et al., 2009). These changes in gene expression patterns have been associated with multiple epigenetic modifications both on a genome-wide level and to specific target genes (for a full review, see Kundakovic and Champagne, 2015). Although the majority of work has focused on the effects of early-life experiences on the hippocampus, there is also evidence for altered epigenetic states in the prefrontal cortex (Roth et al., 2009) and hypothalamus (Murgatroyd et al., 2009; Peña et al., 2013). Weaver et al. (2004) were the first to link differences in maternal care to levels of GR promoter methylation. Analogous changes in methylation after early-life stress have been sought in humans (McGowan et al., 2009; Naumova et al., 2012; Suderman et al., 2014). Yet, it is unclear if DNA methylation, argued by Weaver to be a mechanism for lasting changes, is a cause or consequence of gene expression changes. Changes in gene expression in neuronal populations that drive the function of these neurons can be triggered and maintained via numerous mechanisms. Initiation of transcriptional changes is often secondary to transcription factors (Peña et al., 2017; Singh-Taylor et al., 2017), which might be activated in response to early-life sensory signals and/or changes in calcium entry to the cell resulting from changes in synaptic numbers (Chen et al., 2017). The mechanisms for stable changes in the chromatin that endure for life (and even transgenerationally, Chan et al., 2018) are complex. In addition to these epigenetic changes, there is also some evidence for a role for altered miRNA expression in encoding stress resilience or vulnerabilities from the early-life environment (Bai et al., 2012; Zhang et al., 2013), which may also be heritable across generations (Rodgers et al., 2013; Gapp et al., 2014; de Castro Barbosa et al., 2016; Short et al., 2016; Short et al., 2017). Histone modifications are also likely to play a role, and multiple histone modifications have been associated with differences in early-life experience (Weaver et al., 2004; Peña et al., 2017). These types of chromatin modifications follow both early-life adversity (see above) and beneficial early-life experiences. Repressive histone modifications were recently observed after augmented early-life experience (Singh-Taylor et al., 2017). In this instance, large-scale epigenetic changes were initiated by increases in the function of a transcriptional repressor, NRSF. Later in life, NRSF binding to target genes was no longer observed. Rather, there

174

12. Mechanisms by which early-life experiences promote enduring stress resilience or vulnerability

FIGURE 12.1 A unifying theoretical framework for how early-life experiences can induce long-term changes in behavior. The inciting event is the experience of early-life adversity (green) or repeated, predictable barrages of maternal care (pink), represented in the overlapping circles. Changes in early-life experiences cause a cascade of changes acutely during the perinatal period that results in altered neuronal development and changes in gene expression, which are maintained long-term via epigenetic modifications of the chromatin (represented in the dark grey inner concentric circle). These molecular- and cellular-level changes build upon each other to create altered synaptic connectivity and circuit development at the level of the network, ultimately resulting in the observed alterations in cognition, emotion, and pleasure/reward (represented by the three nodes within the light grey outer concentric circle). Adapted from Bolton, J.L., Molet, J., Ivy, A., Baram, T.Z., 2017. New insights into early-life stress and behavioral outcomes. Current Opinion in Behavioral Sciences 14, 133e139 with permission.

were increases in histone modifications associated with repression of these target genes, including Crh (Singh-Taylor et al., 2017). These results suggest a transition of epigenetic states across the life span in response to changes in the early-life experiences.

Conclusions Early-life experiences modulate risk and resilience to stress-related emotional and cognitive disorders in adulthood. The mechanisms by which experiences during the sensitive developmental period early in life translate into enduring molecular, cellular circuit, and behavioral phenotypes are emerging (Fig. 12.1). This chapter reviews available knowledge. It proposes a unified mechanistic scenario, where patterns of sensory input from the mother influence the number and function of synapses onto stress-sensitive neurons (in analogy to similar processes in visual and auditory systems). Synapse changes regulate transcriptional and epigenetic programming in distinct neuronal populations, which modulate how these

References

175

neurons wire together into circuits and the levels of expression of numerous genes. Together, the altered circuitry and altered neuronal behavior in response to future stimuli promote a phenotype of resilience or vulnerability to stressful signals throughout lifedand perhaps across generations. This framework requires much additional work to affirm or refute. However, it provides a common mechanistic understanding for the enduring consequences of both adverse and beneficial early-life experiences, leading to resilience and vulnerability, respectively, to stress-related emotional and cognitive disorders.

Acknowledgments This work was supported by the National Institutes of Health (grant numbers R01MH073136, R01NS028912, P50MH096889) and the George E. Hewitt Foundation for Medical Research.

References Aisa, B., Tordera, R., Lasheras, B., Del Río, J., Ramírez, M.J., 2007. Cognitive impairment associated to HPA axis hyperactivity after maternal separation in rats. Psychoneuroendocrinology 32 (3), 256e266. Alfarez, D.N., De Simoni, A., Velzing, E.H., Bracey, E., Joëls, M., Edwards, F.A., Krugers, H.J., 2009. Corticosterone reduces dendritic complexity in developing hippocampal CA1 neurons. Hippocampus 19 (9), 828e836. Avishai-Eliner, S., Eghbal-Ahmadi, M., Tabachnik, E., Brunson, K.L., Baram, T.Z., 2001. Down-regulation of hypothalamic corticotrophin releasing hormone messenger ribonucleic acid (mRNA) preceeds early life experience induced changes in hippocampal glucocorticoid receptor mRNA. Endocrinology 142, 89e97. Bai, M., Zhu, X., Zhang, Y., Zhang, S., Zhang, L., Xue, L., Yi, J., Yao, S., Zhang, X., 2012. ‘Abnormal hippocampal BDNF and miR-16 expression is associated with depression-like behaviors induced by stress during early life’ J Homberg (ed). PLoS One 7 (10), e46921. Bale, T.L., 2015. Epigenetic and transgenerational reprogramming of brain development. Nature Reviews Neuroscience 16 (4), 332e344. Bale, T.L., Baram, T.Z., Brown, A.S., Goldstein, J.M., Insel, T.R., McCarthy, M.M., Nemeroff, C.B., Reyes, T.M., Simerly, R.B., Susser, E.S., Nestler, E.J., 2010. Early life programming and neurodevelopmental disorders. Biological Psychiatry 68 (4), 314e319. Baram, T.Z., Davis, E.P., Obenaus, A., Sandman, C.A., Small, S.L., Solodkin, A., Stern, H., 2012. Fragmentation and unpredictability of early-life experience in mental disorders. American Journal of Psychiatry 169 (9), 907e915. Bath, K.G., Manzano-Nieves, G., Goodwill, H., 2016. Early life stress accelerates behavioral and neural maturation of the hippocampus in male mice. Hormones and Behavior 82, 64e71. van Bodegom, M., Homberg, J.R., Henckens, M.J.A.G., 2017. Modulation of the hypothalamic-pituitary-adrenal Axis by early life stress exposure. Frontiers in Cellular Neuroscience 11 (April), 1e33. Bogdan, R., Hariri, A.R., 2012. Neural embedding of stress reactivity. Nature Neuroscience 15 (12), 1605e1607. Bolton, J.L., Ruiz, C., Rismanchi, N., Sanchez, G., Castillo, E., Huang, J., Cross, C., Baram, T.Z., Mahler, S.V., 2019. Early-life adversity facilitates acquisition of cocaine self-administration and induces persistent anhedonia. Neurobiology of Stress. Bolton, J.L., Molet, J., Ivy, A., Baram, T.Z., 2017. New insights into early-life stress and behavioral outcomes. Current Opinion in Behavioral Sciences 14, 133e139. Bolton, J.L., Molet, J., Regev, L., Chen, Y., Rismanchi, N., Haddad, E., Yang, D.Z., Obenaus, A., Baram, T.Z., 2018. Anhedonia following early-life adversity involves aberrant interaction of reward and anxiety circuits and is reversed by partial silencing of amygdala corticotropin-releasing hormone gene. Biological Psychiatry 83 (2), 137e147. Bota, M., Swanson, L.W., 2007. The neuron classification problem. Brain Research Reviews 56 (1), 79e88. Bowlby, J., 1950. Research into the origins of delinquent behaviour. British Medical Journal 1 (4653), 570e573. Bredy, T., Humpartzoomian, R., Cain, D., Meaney, M., 2003. Partial reversal of the effect of maternal care on cognitive function through environmental enrichment. Neuroscience 118 (2), 571e576.

176

12. Mechanisms by which early-life experiences promote enduring stress resilience or vulnerability

Bremner, J.D., Southwick, S.M., Johnson, D.R., Yehuda, R., Charney, D.S., 1993. Childhood physical abuse and combat-related posttraumatic stress disorder in Vietnam veterans. American Journal of Psychiatry 150 (2), 235e239. Brown, A.S., Susser, E.S., Lin, S.P., Neugebauer, R., Gorman, J.M., 1995. Increased risk of affective disorders in males after second trimester prenatal exposure to the Dutch hunger winter of 1944-45. British Journal of Psychiatry: Journal of Mental Science 166 (5), 601e606. Brunson, K.L., Kramár, E., Lin, B., Chen, Y., Colgin, L.L., Yanagihara, T.K., Lynch, G., Baram, T.Z., 2005. Mechanisms of late-onset cognitive decline after early-life stress. Journal of Neuroscience 25 (41), 9328e9338. Burghy, C.A., Stodola, D.E., Ruttle, P.L., Molloy, E.K., Armstrong, J.M., Oler, J.A., Fox, M.E., Hayes, A.S., Kalin, N.H., Essex, M.J., Davidson, R.J., Birn, R.M., 2012. Developmental pathways to amygdala-prefrontal function and internalizing symptoms in adolescence. Nature Neuroscience 15 (12), 1736e1741. de Castro Barbosa, T., Ingerslev, L.R., Alm, P.S., Versteyhe, S., Massart, J., Rasmussen, M., Donkin, I., Sjögren, R., Mudry, J.M., Vetterli, L., Gupta, S., Krook, A., Zierath, J.R., Barrès, R., 2016. High-fat diet reprograms the epigenome of rat spermatozoa and transgenerationally affects metabolism of the offspring. Molecular Metabolism 5 (3), 184e197. Champagne, F.A., 2008. Epigenetic mechanisms and the transgenerational effects of maternal care. Frontiers in Neuroendocrinology 29 (3), 386e397. Chan, J.C., Nugent, B.M., Bale, T.L., May 15, 2018. Parental advisory: maternal and paternal stress can impact offspring neurodevelopment. Biological Psychiatry 83 (10), 886e894. Chen, Y., Baram, T.Z., 2016. Toward understanding how early-life stress reprograms cognitive and emotional brain networks. Neuropsychopharmacology 41 (1), 197e206. Chen, Y., Bender, R.A., Brunson, K.L., Pomper, J.K., Grigoriadis, D.E., Wurst, W., Baram, T.Z., 2004. Modulation of dendritic differentiation by corticotropin-releasing factor in the developing hippocampus. Proceedings of the National Academy of Sciences of the United States of America 101 (44), 15782e15787. Chen, J., Evans, A.N., Liu, Y., Honda, M., Saavedra, J.M., Aguilera, G., 2012. Maternal deprivation in rats is associated with corticotrophin-releasing hormone (CRH) promoter Hypomethylation and enhances CRH transcriptional responses to stress in adulthood. Journal of Neuroendocrinology 24 (7), 1055e1064. Chen, Y., Kramár, E.A., Chen, L.Y., Babayan, A.H., Andres, A.L., Gall, C.M., Lynch, G., Baram, T.Z., 2013. Impairment of synaptic plasticity by the stress mediator CRH involves selective destruction of thin dendritic spines via RhoA signaling. Molecular Psychiatry 18 (4), 485e496. Chen, L.-F., Zhou, A.S., West, A.E., 2017. Transcribing the connectome: roles for transcription factors and chromatin regulators in activity-dependent synapse development. Journal of Neurophysiology 118 (2), 755e770. Malter Cohen, M., Jing, D., Yang, R.R., Tottenham, N., Lee, F.S., Casey, B.J., 2013. Early-life stress has persistent effects on amygdala function and development in mice and humans. Proceedings of the National Academy of Sciences of the United States of America 110 (45), 18274e18278. Davis, E.P., Stout, S.A., Molet, J., Vegetabile, B., Glynn, L.M., Sandman, C.A., Heins, K., Stern, H., Baram, T.Z., 2017. Exposure to unpredictable maternal sensory signals influences cognitive development across species. Proceedings of the National Academy of Sciences 114 (39), 10390e10395. Drury, S.S., Howell, B.R., Jones, C., Esteves, K., Morin, E., Schlesinger, R., Meyer, J.S., Baker, K., Sanchez, M.M., 2017. Shaping long-term primate development: telomere length trajectory as an indicator of early maternal maltreatment and predictor of future physiologic regulation. Development and Psychopathology 29 (5), 1539e1551. Dubé, C.M., Molet, J., Singh-Taylor, A., Ivy, A., Maras, P.M., Baram, T.Z., 2015. Hyper-excitability and epilepsy generated by chronic early-life stress. Neurobiology of Stress 2, 10e19. Dudley, K.J., Li, X., Kobor, M.S., Kippin, T.E., Bredy, T.W., 2011. Epigenetic mechanisms mediating vulnerability and resilience to psychiatric disorders. Neuroscience & Biobehavioral Reviews 35 (7), 1544e1551. Eghbal-Ahmadi, M., Avishai-Eliner, S., Hatalski, C.G., Baram, T.Z., 1999. Differential regulation of the expression of corticotropin-releasing factor receptor type 2 (CRF2) in hypothalamus and amygdala of the immature rat by sensory input and food intake. Journal of Neuroscience 19 (10), 3982e3991. Eriksson, M., Räikkönen, K., Eriksson, J.G., 2014. Early life stress and later health outcomes-findings from the Helsinki Birth Cohort Study. American Journal of Human Biology 26 (2), 111e116. Fenoglio, K.A., Brunson, K.L., Avishai-Eliner, S., Chen, Y., Baram, T.Z., 2004. Region-specific onset of handlinginduced changes in corticotropin-releasing factor and glucocorticoid receptor expression. Endocrinology 145 (6), 2702e2706.

References

177

Fenoglio, K.A., Brunson, K.L., Avishai-Eliner, S., Stone, B.A., Kapadia, B.J., Baram, T.Z., 2005. Enduring, handlingevoked enhancement of hippocampal memory function and glucocorticoid receptor expression involves activation of the corticotropin-releasing factor type 1 receptor. Endocrinology 146 (9), 4090e4096. Fenoglio, K.A., Chen, Y., Baram, T.Z., 2006. Neuroplasticity of the hypothalamic-pituitary-adrenal axis early in life requires recurrent recruitment of stress-regulating brain regions. Journal of Neuroscience 26 (9), 2434e2442. Fox, S.E., Levitt, P., Nelson III, C.A., 2010. How the timing and quality of early experiences influence the development of brain architecture. Child Development 81 (1), 28e40. Gapp, K., Jawaid, A., Sarkies, P., Bohacek, J., Pelczar, P., Prados, J., Farinelli, L., Miska, E., Mansuy, I.M., 2014. Implication of sperm RNAs in transgenerational inheritance of the effects of early trauma in mice. Nature Neuroscience 17 (5), 667e669. Gilles, E.E., Schultz, L., Baram, T.Z., 1996. Abnormal corticosterone regulation in an immature rat model of continuous chronic stress. Pediatric Neurology 15 (2), 114e119. Gray, J.D., Kogan, J.F., Marrocco, J., McEwen, B.S., 2017. Genomic and epigenomic mechanisms of glucocorticoids in the brain. Nature Reviews Endocrinology 13 (11), 661e673. Gunn, B.G., Cunningham, L., Cooper, M.A., Corteen, N.L., Seifi, M., Swinny, J.D., Lambert, J.J., Belelli, D., 2013. Dysfunctional astrocytic and synaptic regulation of hypothalamic glutamatergic transmission in a mouse model of early-life adversity: relevance to neurosteroids and programming of the stress response. Journal of Neuroscience 33 (50), 19534e19554. Gunnar, M.R., 2010. Reversing the effects of early deprivation after infancy: giving children families may not be enough. Frontiers in Neuroscience 4, 170. van Hasselt, F.N., Cornelisse, S., Yuan Zhang, T., Meaney, M.J., Velzing, E.H., Krugers, H.J., Joëls, M., 2012. Adult hippocampal glucocorticoid receptor expression and dentate synaptic plasticity correlate with maternal care received by individuals early in life. Hippocampus 22 (2), 255e266. Hill, R.A., Klug, M., Kiss Von Soly, S., Binder, M.D., Hannan, A.J., van den Buuse, M., 2014. Sex-specific disruptions in spatial memory and anhedonia in a “two hit” rat model correspond with alterations in hippocampal brainderived neurotrophic factor expression and signaling. Hippocampus 24 (10), 1197e1211. Hodel, A.S., Hunt, R.H., Cowell, R.A., Van Den Heuvel, S.E., Gunnar, M.R., Thomas, K.M., 2015. Duration of early adversity and structural brain development in post-institutionalized adolescents. NeuroImage 105, 112e119. Insel, T.R., 2009. Translating scientific opportunity into public health impact. Archives of General Psychiatry 66 (2), 128. Ivy, A.S., Brunson, K.L., Sandman, C., Baram, T.Z., 2008. Dysfunctional nurturing behavior in rat dams with limited access to nesting material: a clinically relevant model for early-life stress. Neuroscience 154 (3), 1132e1142. Ivy, A.S., Rex, C.S., Chen, Y., Dubé, C., Maras, P.M., Grigoriadis, D.E., Gall, C.M., Lynch, G., Baram, T.Z., 2010. Hippocampal dysfunction and cognitive impairments provoked by chronic early-life stress involve excessive activation of CRH receptors. Journal of Neuroscience 30 (39), 13005e13015. Jafari, M., Seese, R.R., Babayan, A.H., Gall, C.M., Lauterborn, J.C., 2012. Glucocorticoid receptors are localized to dendritic spines and influence local actin signaling. Molecular Neurobiology 46 (2), 304e315. Joëls, M., Baram, T.Z., 2009. The neuro-symphony of stress. Nature Reviews Neuroscience 10 (6), 459e466. Juul, S.E., Beyer, R.P., Bammler, T.K., Farin, F.M., Gleason, C.A., 2011. Effects of neonatal stress and morphine on murine hippocampal gene expression. Pediatric Research 69 (4), 285e292. Kaplan, G.A., Turrell, G., Lynch, J.W., Everson, S.A., Helkala, E.-L.L., Salonen, J.T., 2001. Childhood socioeconomic position and cognitive function in adulthood. International Journal of Epidemiology 30 (2), 256e263. Karsten, C.A., Baram, T.Z., 2013. How does a neuron “know” to modulate its epigenetic machinery in response to early-life environment/experience? Frontiers in Psychiatry 4 (August), 1e5. Kessler, R.C., Demler, O., Frank, R.G., Olfson, M., Pincus, H.A., Walters, E.E., Wang, P., Wells, K.B., Zaslavsky, A.M., 2005. Prevalence and treatment of mental disorders, 1990 to 2003. New England Journal of Medicine 352 (24), 2515e2523. Korosi, A., 2009. ‘The pathways from mother’s love to baby’s future’. Frontiers in Behavioral Neuroscience 3, 1e8. Korosi, A., Baram, T.Z., 2008. The central corticotropin releasing factor system during development and adulthood. European Journal of Pharmacology 583 (2), 204e214. Korosi, A., Shanabrough, M., McClelland, S., Liu, Z.-W., Borok, E., Gao, X.-B., Horvath, T.L., Baram, T.Z., 2010. Earlylife experience reduces excitation to stress-responsive hypothalamic neurons and reprograms the expression of corticotropin-releasing hormone. Journal of Neuroscience 30 (2), 703e713.

178

12. Mechanisms by which early-life experiences promote enduring stress resilience or vulnerability

Kundakovic, M., Champagne, F.A., 2015. Early-life experience, epigenetics, and the developing brain. Neuropsychopharmacology 40 (1), 141e153. Lesuis, S., Maurin, H., Borghgraef, P., Lucassen, P., Van Leuven, F., Krugers, H., 2016. Positive and negative early life experiences differentially modulate long term survival and amyloid protein levels in a mouse model of Alzheimer’s disease. Oncotarget 7 (26), 39118e39135. Levine, S., 1957. Infantile experience and resistance to physiological stress. Science 126 (3270), 405e405. Liston, C., Cichon, J.M., Jeanneteau, F., Jia, Z., Chao, M.V., Gan, W.-B., 2013. Circadian glucocorticoid oscillations promote learning-dependent synapse formation and maintenance. Nature Neuroscience 16 (6), 698e705. Liu, D., Diorio, J., Tannenbaum, B., Caldji, C., Francis, D., Freedman, A., Sharma, S., Pearson, D., Plotsky, P.M., Meaney, M.J., 1997. Maternal care, hippocampal glucocorticoid receptors, and hypothalamic-pituitary-adrenal responses to stress. Science 277 (5332), 1659e1662. Lucassen, P.J., Naninck, E.F.G., van Goudoever, J.B., Fitzsimons, C., Joels, M., Korosi, A., 2013. Perinatal programming of adult hippocampal structure and function; emerging roles of stress, nutrition and epigenetics. Trends in Neurosciences 36 (11), 621e631. Lupien, S.J., McEwen, B.S., Gunnar, M.R., Heim, C., 2009. Effects of stress throughout the lifespan on the brain, behaviour and cognition. Nature Reviews Neuroscience 10 (6), 434e445. Lyons, D., 2009. Developmental cascades linking stress inoculation, arousal regulation, and resilience. Frontiers in Behavioral Neuroscience 3, 32. Maestripieri, D., Higley, J.D., Lindell, S.G., Newman, T.K., McCormack, K.M., Sanchez, M.M., 2006. Early maternal rejection affects the development of monoaminergic systems and adult abusive parenting in rhesus macaques (Macaca mulatta). Behavioral Neuroscience 120 (5), 1017e1024. Magarinõs, A.M., McEwen, B.S., 1995. Stress-induced atrophy of apical dendrites of hippocampal CA3c neurons: involvement of glucocorticoid secretion and excitatory amino acid receptors. Neuroscience 69 (1), 89e98. Makino, S., Gold, P.W., Schulkin, J., 1994. Corticosterone effects on corticotropin-releasing hormone mRNA in the central nucleus of the amygdala and the parvocellular region of the paraventricular nucleus of the hypothalamus. Brain Research 640 (1e2), 105e112. Maras, P.M., Baram, T.Z., 2012. Sculpting the hippocampus from within: stress, spines, and CRH. Trends in Neurosciences 35 (5), 315e324. Mason, W.A., Harlow, H.F., 1958. Performance of infant rhesus monkeys on a spatial discrimination problem. Journal of Comparative & Physiological Psychology 51 (1), 71e74. McEwen, B.S., 2003. Early life influences on life-long patterns of behavior and health. Mental Retardation and Developmental Disabilities Research Reviews 9 (3), 149e154. McGowan, P.O., Sasaki, A., D’Alessio, A.C., Dymov, S., Labonté, B., Szyf, M., Turecki, G., Meaney, M.J., Labonte, B., Szyf, M., Turecki, G., Meaney, M.J., 2009. Epigenetic regulation of the glucocorticoid receptor in human brain associates with childhood abuse. Nature Neuroscience 12 (3), 342e348. Meaney, M.J., Szyf, M., 2005. Maternal care as a model for experience-dependent chromatin plasticity? Trends in Neurosciences 28 (9), 456e463. Millstein, R.A., Holmes, A., 2007. Effects of repeated maternal separation on anxiety- and depression-related phenotypes in different mouse strains. Neuroscience & Biobehavioral Reviews 31 (1), 3e17. Molet, J., Maras, P.M., Avishai-Eliner, S., Baram, T.Z., 2014. Naturalistic rodent models of chronic early-life stress. Developmental Psychobiology 56 (8), 1675e1688. Molet, J., Heins, K., Zhuo, X., Mei, Y.T., Regev, L., Baram, T.Z., Stern, H., 2016a. Fragmentation and high entropy of neonatal experience predict adolescent emotional outcome. Translational Psychiatry 6 (1), e702. Molet, J., Maras, P.M., Kinney-Lang, E., Harris, N.G., Rashid, F., Ivy, A.S., Solodkin, A., Obenaus, A., Baram, T.Z., 2016b. MRI uncovers disrupted hippocampal microstructure that underlies memory impairments after earlylife adversity. Hippocampus 26 (12), 1618e1632. Dalle Molle, R., Portella, A.K., Goldani, M.Z., Kapczinski, F.P., Leistner-Segala, S., Salum, G.A., Manfro, G.G., Silveira, P.P., 2012. Associations between parenting behavior and anxiety in a rodent model and a clinical sample: relationship to peripheral BDNF levels. Translational Psychiatry 2 (11), e195. Moriceau, S., Shionoya, K., Jakubs, K., Sullivan, R.M., 2009. Early-life stress disrupts attachment learning: the role of amygdala corticosterone, locus ceruleus corticotropin releasing hormone, and olfactory bulb norepinephrine. Journal of Neuroscience 29 (50), 15745e15755.

References

179

Murgatroyd, C., Patchev, A.V., Wu, Y., Micale, V., Bockmuhl, Y., Fischer, D., Holsboer, F., Wotjak, C.T., Almeida, O.F., Spengler, D., 2009. Dynamic DNA methylation programs persistent adverse effects of early-life stress. Nature Neuroscience 12 (12), 1559e1566. Naninck, E.F.G., Hoeijmakers, L., Kakava-Georgiadou, N., Meesters, A., Lazic, S.E., Lucassen, P.J., Korosi, A., 2015. Chronic early life stress alters developmental and adult neurogenesis and impairs cognitive function in mice. Hippocampus 25 (3), 309e328. Naumova, O.Y., Lee, M., Koposov, R., Szyf, M., Dozier, M., Grigorenko, E.L., 2012. Differential patterns of wholegenome DNA methylation in institutionalized children and children raised by their biological parents. Development and Psychopathology 24 (1), 143e155. Nelson, C.A., Zeanah, C.H., Fox, N.A., Marshall, P.J., Smyke, A.T., Guthrie, D., 2007. Cognitive recovery in socially deprived young children: the Bucharest Early Intervention Project. Science 318 (5858). Parker, K.J., Buckmaster, C.L., Sundlass, K., Schatzberg, A.F., Lyons, D.M., 2006. Maternal mediation, stress inoculation, and the development of neuroendocrine stress resistance in primates. Proceedings of the National Academy of Sciences of the United States of America 103 (8), 3000e3005. Peña, C.J., Neugut, Y.D., Champagne, F.A., 2013. Developmental timing of the effects of maternal care on gene expression and epigenetic regulation of hormone receptor levels in female rats. Endocrinology 154 (11), 4340e4351. Peña, C.J., Kronman, H.G., Walker, D.M., Cates, H.M., Bagot, R.C., Purushothaman, I., Issler, O., Loh, Y.E., Leong, T., Kiraly, D.D., Goodman, E., Neve, R.L., Shen, L., Nestler, E.J., 2017. Early life stress confers lifelong stress susceptibility in mice via ventral tegmental area OTX2. Science 356 (6343), 1185e1188. Plotsky, P.M., Meaney, M.J., 1993. Early, postnatal experience alters hypothalamic corticotropin-releasing factor (CRF) mRNA, median eminence CRF content and stress-induced release in adult rats. Molecular Brain Research 18 (3), 195e200. Radley, J.J., Rocher, A.B., Rodriguez, A., Ehlenberger, D.B., Dammann, M., McEwen, B.S., Morrison, J.H., Wearne, S.L., Hof, P.R., 2008. Repeated stress alters dendritic spine morphology in the rat medial prefrontal cortex. The Journal of Comparative Neurology 507 (1), 1141e1150. Raineki, C., Cortés, M.R., Belnoue, L., Sullivan, R.M., 2012. Effects of early-life abuse differ across development: infant social behavior deficits are followed by adolescent depressive-like behaviors mediated by the amygdala. Journal of Neuroscience 32 (22), 7758e7765. Regev, L., Baram, T.Z., 2014. Corticotropin releasing factor in neuroplasticity. Frontiers in Neuroendocrinology 35 (2), 171e179. Rice, C.J., Sandman, C.A., Lenjavi, M.R., Baram, T.Z., 2008. A novel mouse model for acute and long-lasting consequences of early life stress. Endocrinology 149 (10), 4892e4900. Rodgers, A.B., Morgan, C.P., Bronson, S.L., Revello, S., Bale, T.L., 2013. Paternal stress exposure alters sperm microRNA content and reprograms offspring HPA stress axis regulation. Journal of Neuroscience 33 (21), 9003e9012. Roth, T.L., Lubin, F.D., Funk, A.J., Sweatt, J.D., 2009. Lasting epigenetic influence of early-life adversity on the BDNF gene. Biological Psychiatry 65 (9), 760e769. Russo, S.J., Murrough, J.W., Han, M.-H., Charney, D.S., Nestler, E.J., 2012. Neurobiology of resilience. Nature Neuroscience 15 (11), 1475e1484. Shalev, U., Kafkafi, N., 2002. Repeated maternal separation does not alter sucrose-reinforced and open-field behaviors. Pharmacology Biochemistry and Behavior 73 (1), 115e122. Short, A.K., Fennell, K.A., Perreau, V.M., Fox, A., O’Bryan, M.K., Kim, J.H., Bredy, T.W., Pang, T.Y., Hannan, A.J., 2016. Elevated paternal glucocorticoid exposure alters the small noncoding RNA profile in sperm and modifies anxiety and depressive phenotypes in the offspring. Translational Psychiatry 6 (6), e837. Short, A.K., Yeshurun, S., Powell, R., Perreau, V.M., Fox, A., Kim, J.H., Pang, T.Y., Hannan, A.J., 2017. Exercise alters mouse sperm small noncoding RNAs and induces a transgenerational modification of male offspring conditioned fear and anxiety. Translational Psychiatry 7 (5), e1114. Silveira, P.P., Portella, A.K., Clemente, Z., Gamaro, G.D., Dalmaz, C., 2005. The effect of neonatal handling on adult feeding behavior is not an anxiety-like behavior. International Journal of Developmental Neuroscience 23 (1), 93e99. Singh-Taylor, A., Korosi, A., Molet, J., Gunn, B.G., Baram, T.Z., 2015. Synaptic rewiring of stress-sensitive neurons by early-life experience: a mechanism for resilience? Neurobiology of Stress 1 (1), 109e115.

180

12. Mechanisms by which early-life experiences promote enduring stress resilience or vulnerability

Singh-Taylor, A., Molet, J., Jiang, S., Korosi, A., Bolton, J.L., Noam, Y., Simeone, K., Cope, J., Chen, Y., Mortazavi, A., Baram, T.Z., 2017. NRSF-dependent epigenetic mechanisms contribute to programming of stress-sensitive neurons by neonatal experience, promoting resilience. Molecular Psychiatry 0 1e10. Slattery, D.A., Cryan, J.F., 2012. Using the rat forced swim test to assess antidepressant-like activity in rodents. Nature Protocols 7 (6), 1009e1014. Suderman, M., Borghol, N., Pappas, J.J., Pinto Pereira, S.M., Pembrey, M., Hertzman, C., Power, C., Szyf, M., 2014. Childhood abuse is associated with methylation of multiple loci in adult DNA. BMC Medical Genomics 7 (1), 13. Tang, A.C., 2001. Neonatal exposure to novel environment enhances hippocampal-dependent memory function during infancy and adulthood. Learning & Memory 8 (5), 257e264. Walker, C.D., Bath, K.G., Joels, M., Korosi, A., Larauche, M., Lucassen, P.J., Morris, M.J., Raineki, C., Roth, T.L., Sullivan, R.M., Taché, Y., Baram, T.Z., 2017. Chronic early life stress induced by limited bedding and nesting (LBN) material in rodents: critical considerations of methodology, outcomes and translational potential. Stress: The International Journal on the Biology of Stress 20 (5), 421e448. Wang, X.-D., Rammes, G., Kraev, I., Wolf, M., Liebl, C., Scharf, S.H., Rice, C.J., Wurst, W., Holsboer, F., Deussing, J.M., Baram, T.Z., Stewart, M.G., Müller, M.B., Schmidt, M.V., 2011. ‘Forebrain CRF₁ modulates early-life stress-programmed cognitive deficits’. Journal of Neuroscience 31 (38), 13625e13634. Wang, A., Nie, W., Li, H., Hou, Y., Yu, Z., Fan, Q., Sun, R., 2014. Epigenetic upregulation of corticotrophin-releasing hormone mediates postnatal maternal separation-induced memory deficiency’ SD ginsberg (ed). PLoS One 9 (4), e94394. Weaver, I.C., Cervoni, N., Champagne, F.A., D’Alessio, A.C., Sharma, S., Seckl, J.R., Dymov, S., Szyf, M., Meaney, M.J., 2004. Epigenetic programming by maternal behavior. Nature Neuroscience 7 (8), 847e854. Whitton, A.E., Treadway, M.T., Pizzagalli, D.A., 2015. Reward processing dysfunction in major depression, bipolar disorder and schizophrenia. Current Opinion in Psychiatry 28 (1), 7e12. Zhang, Y., Zhu, X., Bai, M., Zhang, L., Xue, L., Yi, J., 2013. Maternal deprivation enhances behavioral vulnerability to stress associated with miR-504 expression in nucleus accumbens of rats. PLoS One 8 (7), e69934.

C H A P T E R

13

Child abuse and neglect: stress responsivity and resilience 1

Shariful A. Syed1, Matthew Cranshaw2, Charles B. Nemeroff1

Department of Psychiatry and Behavioral Sciences, Miller School of Medicine, Miami, FL, United States; 2University of Miami, Miller School of Medicine, Miami, FL, United States

Child abuse and neglect is one of the most prevalent forms of trauma experienced in the modern world (Anda et al., 2006). In the United States alone there were approximately 4 million reports of child maltreatment in 2015 (increased from 3.4 million in 2012), including 1,585 fatalities, with only 1 in 10 victims receiving any form of postabuse care/service. Furthermore, since 2008, the overall incidence of childhood maltreatment appears to be steadily increasing. Of the child fatalities, 72.9% were neglected children, 43.9% physically abused, and 1.2% sexually abused (Health USDo and Human S, 2015). National reports (Fig. 13.1) have consistently shown that early-life trauma predisposes individuals to develop a number of psychiatric syndromes, particular mood and anxiety disorders, and as such, is a significant public health problem (Molnar et al., 2001). The mechanisms by which various forms of child abuse increase the risk of developing psychiatric disorders are believed to stem from their profound short- and long-term effects on the central nervous system (CNS) and a multitude of peripheral organ systems (Heim et al., 2000a). The neurobiological mechanisms that mediate the consequences of early developmental stress have been studied in humans and laboratory animals. The increased rates of several psychiatric disorders after exposure to early-life stress (ELS) suggest a persistent sensitivity to the effects of stress in later life (Heim and Binder, 2012). More specifically, child abuse and neglect have been posited to permanently sensitize and dysregulate various components of the stress response, both centrally and peripherally. Although the goal of this book is to generate a neurobiological paradigm of stress resilience, in this chapter we focus on one of the most pivotal aspects of this paradigm, namely stress responsivity and how we may discern resilience mechanisms from the stress neurobiology of childhood abuse and neglect. To begin, we present the functional definition of “stress responsivity” to be “variability in reaction to stressful stimuli.” We have chosen such a definition, as it appears to be congruent Stress Resilience https://doi.org/10.1016/B978-0-12-813983-7.00013-6

181

Copyright © 2020 Elsevier Inc. All rights reserved.

182

13. Child abuse and neglect: stress responsivity and resilience

FIGURE 13.1 Number of cases of child abuse in the United States in 2015, according to the type of abuse. Adapted from the U.S. Department of Health and Human Services.

with the theory of evolution as well as the physiology subserving the human stress response. A challenge, which we will elaborate on later in the chapter, although necessary to address from the start, is the difficulty of developing a generalizable definition for “resilience.” To date, the field of stress neurobiology has largely presumed it to mean the absence of psychopathology after extreme stress. This is, in part, a product of the simple fact that we are still in the early stages of “stress resilience” neurobiological research. As the scientific community continues to further elucidate mechanisms via advancement in methodological approaches as well as innovative methods of investigation, we are optimistic that a more refined and comprehensible definition will be operationalized. Before delving into the various mechanisms involved in stress responsivity, a brief overview of the two major systems, which mediate the human stress response, is necessary, namely, the hypothalamic-pituitary-adrenal (HPA) axis and sympathetic adrenomedullary (SAM) system. Before beginning this discussion, we would highlight a fact of the utmost importance. The age group most vulnerable to abuse (neonatesdchildren aged 2 years) is also the group abused the most (Health USDo and Human S, 2015).

Stress responsivity physiology The two main components of the mammalian stress response are the SAM system and the HPA axis (Gunnar and Quevedo, 2007). CNS circuits involving areas of the prefrontal cortex, hippocampus, amygdala, hypothalamic, and brain stem nuclei modulate both systems. Corticotropin-releasing factor (CRF)eproducing neurons oversee the entire mammalian stress response, coordinating the autonomic, endocrine, immune, and behavioral responses to stress (Arborelius et al., 1999). The highest concentrations of CRF are found in the paraventricular nucleus (PVN) of the hypothalamus, which primarily regulates the HPA axis

Stress responsivity physiology

183

response to stress (Antoni et al., 1983). CRF-producing neurons located in the central nucleus of the amygdala are involved in processing emotional stress responses and the SAM response as well. CRF neurons in the central nucleus of the amygdala project to locus coeruleus norepinephrine cells, which in turn project to the lateral thalamus, leading to subsequent activation of the sympathetic preganglionic neurons that ultimately stimulate release of epinephrine from the adrenal medulla. CRF cells of the central nucleus of amygdala are involved in stress-induced activation of the HPA axis (Shekhar et al., 2005), via an indirect pathway through the bed nucleus of the stria terminalis, where CRF neuronal projections innervate the PVN neurons of the hypothalamus (Herman and Cullinan, 1997; Herman et al., 2002; Swanson and Sawchenko, 1983). Following activation of the HPA axis, CRF is released from the PVN in to the adenohypophysial-portal circulation from nerve terminals in the median eminence where it stimulates adrenocorticotropin hormone (ACTH) release from the anterior pituitary. ACTH in turn stimulates release of glucocorticoids (GCs) from the adrenal cortex (Gutman and Nemeroff, 2002). Able to permeate the blood-brain barrier, GCs reduce activation of the HPA axis via stimulation of GC receptors (GRs) within the hippocampus, hypothalamus, and anterior pituitary (Jacobson and Sapolsky, 1991). The critical role of amygdalar CRF has brought to attention the widespread localization of CRF receptors throughout the CNS and their converging pathways in orchestrating stress reactions (Swiergiel et al., 1993; Nemeroff, 1996). Two G proteinecoupled subtypes of CRF receptors, CRFR1 and CRFR2, have been found in the anterior pituitary, as well as in subcortical and cortical brain areas (Chalmers et al., 1996; Steckler and Holsboer, 1999). In general, the stress response appears to be mediated largely by CRFR1 receptors, whereas CRFR2 activation appears to actually diminish the stress response (see Chapter 16 for more information). The response to psychosocial stress, of which ELS represents a specific subtype, also involves “higher appraisal” by cortical and subcortical regions of brain containing CRFR1 receptors, namely, cingulate cortex, orbital/medial prefrontal cortex, and hippocampus (Bale and Vale, 2004); all these areas comprise part of the converging pathways described above. Much evidence points to the role for CRF as a neurotransmitter coordinating immune, autonomic, endocrine, and behavioral stress responses, supported by the finding that CRFR1 receptors are more abundant in corticolimbic pathways that mediate fear- and anxiety-related behaviors (Sanchez et al., 1999). With this basic framework of stress response neurobiology, the concept of “stress responsivity” may be viewed through a model of posttraumatic stress disorder (PTSD). In many ways, as discussed in more detail in other chapters of this book, individuals diagnosed with PTSD exhibit dysregulation of the stress response system. Criteria D for diagnosing PTSD in DSM5 requires that an individual demonstrates “marked alterations in arousal and reactivity associated with traumatic event(s)” in the form of hypervigilance, exaggerated startle response, increased irritability, problems with concentration, and/or sleep disturbance. The fact that many individuals with PTSD have experienced traumatic events that occurred in the form of child abuse and neglect comes as little surprise and further strengthens the argument for using PTSD-derived neurobiological research in developing the construct of “stress responsivity.”

184

13. Child abuse and neglect: stress responsivity and resilience

In the space below, we will examine in further detail the available evidence from human studies on the role of child abuse and neglect that contribute toward a model of “stress responsivity.”

Hypothalamic-pituitary-adrenal axis physiology The HPA axis represents the major neuroendocrine stress response system that serves to adapt the organism to change in life demands and thereby maintain homeostasis (McEwen, 2004). Studies of the influence of ELS on HPA axis activity have shown that the effects of child abuse and neglect are variable in that it is associated with either increased or decreased HPA axis activity. This variability is dependent on several factors including age at the time of the trauma, subtype of abuse/neglect, magnitude, duration, etc.

Childhood maltreatment influence on hypothalamic-pituitary-adrenal/ sympathetic nervous system response to stress Using various validated human stress models of provocative adrenal testing, HPA axis hyperactivity was demonstrated in depressed women and men with ELS by increases in both the ACTH and cortisol response as well as increased cerebrospinal fluid (CSF) CRF concentrations (Heim and Nemeroff, 2001; Heim et al., 2000b, 2002; Carpenter et al., 2004). In an early study, we tested the hypothesis that ELS in humans is associated with persistent sensitization of the HPA axis (Heim et al., 2003). To induce stress, we employed a standardized psychosocial stress protocol, the Trier Social Stress Test (TSST) that consists of public speaking and mental arithmetic tasks in front of an “audience” that has been shown to reliably induce HPA axis and sympathetic nervous system activation (Kirschbaum et al., 1993). Parallel to results from animal models, women with a history of childhood abuse (with and without) current major depressive disorder (MDD) exhibited increased ACTH responses to stress compared with controls. Overall the ACTH response was more than sixfold greater in abused women with current MDD than in controls. These women also demonstrated increased cortisol and heart rate responses to psychosocial stress. Abused women who were not currently depressed exhibited normal cortisol responses, despite their increased ACTH response, perhaps suggesting adrenal adaptation to central sensitization as a marker of resilience against depression after early stress. Depressed women without abuse demonstrated normal neuroendocrine responses. Our findings suggested that HPA axis and autonomic nervous system hyperactivity, likely due to CRF hypersecretion, may be a persistent consequence of childhood abuse that may contribute to the diathesis for adulthood psychopathology (Monroe and Simons, 1991). In a study of patients with MDD and borderline personality disorder, higher baseline and postdexamethasone cortisol concentrations were found in those who had a history of childhood trauma (Fernando et al., 2012). In a nonclinical sample of women with minimal or no current psychopathology, childhood physical abuse was associated with a blunted cortisol response to psychosocial stress task (Carpenter et al., 2011). In another sample of 230 adults without a primary affective disorder,

Glucocorticoid feedback regulation of stress responsivity

185

a history of self-reported childhood emotional abuse predicted a significantly diminished cortisol response after administration of the dexamethasone/CRF test (Carpenter et al., 2009). In contrast, individuals with child abuse have been reported to exhibit reduced basal cortisol levels, as well as a blunted cortisol response to provocative stimuli (Carpenter et al., 2007). Likewise, ELS is well documented to increase the risk for development of PTSD, which is characterized by an “endocrine signature” of GR hypersensitivity and reduced cortisol signaling (Bradley and Blakely, 1997).

Sympathetic nervous system Perceived threat activates the sympathetic (SNS) and parasympathetic (PNS) nervous systems and recurrent high levels of threat exposure, particularly early in life, can significantly affect an individual’s long-term ability to modulate the SNS and PNS response to future stressors (McEwen, 1998). Although the majority of studies have been focused on the HPA axis, others have examined childhood abuse and SNS reactivity. Of these, some report increased SNS reactivity following high levels of family adversity, whereas others observe no such association (Ellis et al., 2005; Oosterman et al., 2010; El-Sheikh, 2005; Elzinga et al., 2008). Women with a history of childhood sexual abuse also demonstrate relatively high SNS activity (Weiss et al., 1999), particularly in response to sexual cues (Rellini and Meston, 2006). Child abuse is associated with maladaptive patterns of cardiovascular reactivity to psychosocial stress in adolescence (McLaughlin et al., 2014). Taken together, these described HPA axis and SNS changes are consistent with an abnormal increase of CNS CRF activity as a function of childhood abuse and neglect. In fact, childhood stress has been suggested to be more predictive of increased CSF CRF concentrations than either a syndromal diagnosis of a depressive disorder or a suicide attempt (De Bellis and Thomas, 2003). Further highlighting the importance of the timing of stressors, Carpenter et al. (2004) reported that a history of adverse life events before age 6 years predicted elevated CSF CRF concentrations better than the diagnosis of MDD (Schoedl et al., 2010).

Glucocorticoid feedback regulation of stress responsivity Enhanced stress responsiveness after childhood trauma might be further influenced by changes in GC-mediated feedback control of the HPA axis. In an initial study, we observed increased suppression of cortisol in a low-dose dexamethasone suppression test in abused women with depression and concurrent PTSD (Newport et al., 2004). Such hypersuppression indicates enhanced sensitivity of the pituitary to negative feedback and is a prominent finding in PTSD, believed to contribute to stress sensitization (Yehuda, 2006). In fact, the results found in this study might be best attributable to comorbidity with PTSD. We sought to determine the effects of childhood abuse on results in the dexamethasone/CRF test in adult men with and without current MDD. Abused men demonstrated markedly increased cortisol

186

13. Child abuse and neglect: stress responsivity and resilience

responses to dexamethasone/CRF administration when compared with nonabused men, regardless of diagnosis. When stratifying groups by MDD and childhood trauma, only those abused men with current MDD, but not depressed men without childhood trauma, demonstrated increased cortisol responses. Increased response was associated with exposure to both sexual and physical abuse and the severity of the abuse (Heim et al., 2008). Importantly, this effect was not attributable to comorbid PTSD. These results suggest that childhood trauma is associated with impaired GC-mediated feedback control of the HPA axis during stimulated conditions (Heim et al., 2008).

Epigenetics of stress responsivity It is firmly established that genetics contribute to the risk for the development of major psychiatric disorders. In addition, child abuse and neglect serve as important risk factors for the development of psychiatric disorders (Agid et al., 1999; Nestler et al., 2002). A novel approach utilized in recent years tests the hypothesis that gene variants may modulate the effect of ELS on the longitudinal risk for mental illness. Diathesis-stress theories of depression suggest that individual’s sensitivity to stressful events depends, in part, on their genotype (Costello et al., 2002). Investigations to date have largely supported this theory, with many studies demonstrating gene  environment (G  E) interactions that predict psychiatric disorder risk. A handful of such genetic polymorphisms are reviewed here: serotonin transporterelinked polymorphic region (5HTTLPR), monoamine oxidase A (MAOA), FK506-binding protein 51 (FKBP51), CRFR1, brain-derived neurotrophic factor (BDNF), and opioid-related nociceptin receptor 1 (OPRL1). Much research has focused on the interaction between serotonin transporter polymorphisms, ELS, and depression. In a pioneering study using the Dunedin cohort, Caspi et al. (2003) were the first to demonstrate an association between depression, ELS, and the 5-HTTLPR genotype. Individuals exposed to childhood maltreatment, possessing the s/s genotype, were shown to have the highest probability of developing a MDD episode and/or exhibit suicidality, followed by the s/l genotype. The l/l genotype was associated with resiliencedno increased risk for depression or suicide even in the presence of severe childhood abuse or neglect. In a general population study, a three-way interaction among childhood abuse  adult traumatic experience  s allele carrier status was found to be associated with higher Beck Depression Inventory-II (BDI-II) scores (Grabe et al., 2012). A meta-analysis by Karg et al. (2011) found strong evidence supporting the association between childhood maltreatment and the s allele and increased stress sensitivity. This G  E discovery leads to an interesting question, namely, whether we can use a patient’s genotype for the serotonin transporter promoter polymorphism, as well as other polymorphisms coupled with a history of child abuse/neglect as criteria for early intervention to prevent the development of MDD in vulnerable individuals. Caspi et al. (2002) were also among the first to suggest that individual differences at a functional polymorphism in the promoter region of the MAOA gene may modulate children’s response to maltreatment.

Stress responsivity neural circuits

187

As noted previously, the association between child abuse and adult PTSD is well established (Bremner et al., 1993). Given that PTSD is strongly associated with long-lasting alterations in HPA axis sensitivity and increased GR sensitivity, a natural extension of GE research has examined whether HPA axis gene candidates mediate the increased susceptibility to PTSD after ELS (Yehuda, 2001; Yehuda et al., 1991). FKBP51 codes for a cochaperone protein that modules signal transduction of the GR. Four FKBP51 SNPs were found to significantly predict the PTSD Symptom Score (PSS) in individuals with a history of child abuse. All four SNPs have been associated with the presence of higher levels of FKBP51, consistent with the physiological mechanisms mediating GC sensitivity (Binder et al., 2008). Bradley et al. (2008) demonstrated that genetic variants of the CRFR1 moderate the effect of child abuse on adult depressive symptoms. Laucht et al. (2013) found that the impact of childhood maltreatment on adult depressive symptoms was higher in individuals with two copies of the CRFR1 TAT haplotype. A haplotype of three SNPs in intron 1 of the CRFR1 gene was associated with a diminished effect of child abuse on adult depressive symptoms (Bradley et al., 2008). Thus, a genotype/haplotype may serve as a predictor of risk/resilience in those with history of child abuse and neglect. Ressler et al. found that variants in the 5-HTTLPR interact with CRFR1 genotypes to predict current adult depressive symptoms. Individuals carrying a “risk” allele in both genes demonstrated more severe depressive symptoms at lower levels of child abuse (Ressler et al., 2010). Another GG interaction with implications of vulnerability to depression is between 5-HTTLPR and BDNF. Meta-analyses have suggested that alteration in serotonergic activity may serve as a prodrome for later changes in neural plasticity of which BDNF is essential (Munafo et al., 2005; Urani et al., 2005). One study suggested that the BDNF Met allele may serve as a protection against the adverse effects associated with the 5-HTTLPR s allele in healthy individuals. However, in maltreated children, the combination of BDNF Met with the 5-HTTLPR s allele was associated with an increased risk for MDD (Kaufman et al., 2006).

Stress responsivity neural circuits That ELS, including child abuse and neglect, produces persistent increases in CSF CRF concentrations, a measure of activity of CRF-containing neural circuits, a hallmark of HPA axis hyperactivity, and dysregulation of corticolimbic circuits places it in a position of fundamental importance in exploring pathogenic mechanisms that may underlie major psychiatric illnesses such as MDD and PTSD. Within the context of ELS, emerging data are all congruent in demonstrating persistent structural and functional changes to CNS structures and circuits including the prefrontal cortex, hippocampus, amygdala, and other cortical/ subcortical areas of brain, with increasing evidence that the ELS-specific subtypes result in specific neuroanatomical alterations. The hippocampus has long been an area of interest for a multitude of reasons, one being that it is known to play a pivotal role in efficient termination of the HPA axis stress response by its rich density of GRs.

188

13. Child abuse and neglect: stress responsivity and resilience

Moreover, hippocampal volume reductions have been repeatedly reported in those suffering from MDD, PTSD, and other psychiatric disorders. Reports of reduced hippocampal volume in depressed women with a history of childhood maltreatment but not in equally depressed women without ELS have also been confirmed by others (Vythilingam et al., 2002; Buss et al., 2007; Frodl et al., 2010) and in a comprehensive meta-analysis (Nanni et al., 2012). Teicher et al. (2012) found that childhood maltreatment was significantly associated with reduced volume in the hippocampal dentate gyrus, subiculum, and subfield CA3. Another study compared depressed patients and age- and sex-matched healthy controls and found that childhood maltreatment, but not depression, was associated with hippocampal atrophy (Opel et al., 2014). Victims of childhood sexual and emotional abuse showed marked thinning in specific areas of cortical representation, respectively, suggesting that type-specific ELS has select effects on neural plasticity that persist into adulthood (Heim et al., 2013). The preeminent role of the amygdala in stress responsivity has appropriately rendered it a central focus in research on mood and anxiety disorders. Both amygdala volume and responsiveness to stressors in those exposed to child abuse and neglect versus controls have been explored. Childhood maltreatment (assessed by the Childhood Trauma Questionnaire) was shown to be positively associated with amygdala responsiveness in a standard emotional face-matching paradigm. This effect was not confounded by recent life stressors, current depression, or sociodemographic factors (Dannlowski et al., 2012).

Stress responsivity and inflammation A neurodegenerative hypothesis of depression and psychiatric disorders of which inflammation is central has started to gain significant support in the stress neurobiology literature (Maes et al., 2009). Briefly, the mechanisms of action of cytokines on the brain include the ability to mediate “sickness” behavior, alterations in serotonergic/glutamatergic/dopaminergic neurotransmission, reduction in neurotrophic factors (e.g., BDNF), and increased neuronal glutamate excitotoxicity, as well as neuronal vulnerability to oxidative reactive species (Maes et al., 1993, 2009, 2011; Raison et al., 2006, 2010; Qin et al., 2007; Irwin and Miller, 2007; Borland and Michael, 2004; Zhu et al., 2010; Neurauter et al., 2008; Felger et al., 2013; Steiner et al., 2011; Raison and Miller, 2003; Krügel et al., 2013). This has refined the original monoamine hypothesis of depression (Reichenberg et al., 2001; Maes, 1995; Capuron et al., 2002; Harrison et al., 2009; Bonaccorso et al., 2002) with a chronic neuroinflammatory and neurodegenerative theory of depression (Maes et al., 2009). Childhood abuse has been convincingly shown to produce a proinflammatory state (Coelho et al., 2014). Individuals with depression and a history of childhood maltreatment were more likely to have elevated C-reactive protein concentrations compared with controls (Danese et al., 2007, 2011). In response to daily stressors, child abuse history moderated levels of IL-6; those with a positive history of childhood abuse had IL-6 levels 2.35 times greater than those without any early abuse history. Childhood abuse was significantly associated with increased NF-kb pathway activity in individuals with PTSD, providing additional pathways to the previously discussed HPA axis alterations in the context of child abuse and neglect (Pace et al., 2012).

Resilient stress responses: CRFR1/OPRL1/5HTLPR/BDNF/NPY/DHEA

189

Stress responsivity and resilience As discussed earlier, the concept of resilience has proven remarkably challenging to operationalize, as it encompasses a variety of behavioral phenotypes, which further complicates the characterization of neurobiological mechanisms in resilient individuals (Russo et al., 2012). If we consider resilience to be an active process of adaptation that precludes the development of psychopathology in the context of extreme duress, then the study of stress responsivity may be one pathway to advancing our understanding of resilience. To date, it is clear that the scientific literature has amassed a considerate database on the neurobiological consequences of child abuse and neglect and its associated cascade of perturbations associated with increased vulnerability to developing affective and anxiety disorders. However, it may be that through the elucidation of “risk neurobiology,” we may indirectly arrive at ways to reduce risk and thus increase resilience. In several animal models and in some human studies, resilience is associated with rapid activation of the stress response and its efficient termination (DeRijk and de Kloet, 2005; De Kloet et al., 2005) and is further characterized by the capacity to constrain stress-induced increases in CRF and cortisol through an elaborate negative feedback system. There are clinical data in select populations that exemplify stress resilience (i.e., military personnel, victims of trauma), of which there are multiple biological factors that appear to play a role: CRFR1, 5HTLLPR, BDNF, neuropeptide Y (NPY), and dehydroepiandrosterone (DHEA) to name a few.

Resilient stress responses: CRFR1/OPRL1/5HTLPR/BDNF/NPY/DHEA Mediating the bulk of the CNS action of CRF, the CRFR1 receptor gene has demonstrated a haplotype of three SNPs in intron 1 of the CRFR1 gene that was associated with a diminished effect of child abuse on adult depressive symptoms (Bradley et al., 2008). Specific CRFR1 polymorphisms appeared to uniquely moderate the effect of child abuse on the prospective risk for depressive symptoms in adulthood. Thus, a genotype/haplotype may serve as a predictor of both risk and resilience in those with history of child abuse and neglect. To add to the ELS-HPA axis gene interaction story, an SNP found in the opioid receptore like 1 (Oprl1) gene in patients with PTSD symptoms after a traumatic event is associated with a self-reported history of childhood trauma (Andero et al., 2013). The same SNP is associated with altered fear learning and fear discrimination, mechanisms including differential amygdala-insula functional connectivity that has been linked to PTSD (Stein et al., 2007). Kaufman et al. (2004) showed that a supportive environment protected children with the s/s serotonin transporter promoter genotype and a history of maltreatment from developing depression. BDNF, a vital growth factor, promotes healthy function of the adult hippocampus. Induction of BDNF has been implicated in relative vulnerability and resilience to stress (Krishnan et al., 2007). NPY is a peptide neurotransmitter that modulates the acute stress response, and laboratory animal studies have provided evidence that increased NPY

190

13. Child abuse and neglect: stress responsivity and resilience

signaling in the central nucleus of the amygdala is associated with lower anxiety levels (Rasmusson et al., 2003). DHEA is released with cortisol from the adrenal cortex, and studies suggest it may play an antiinflammatory/antioxidant role during an acute stress response. DHEA increased under acute stress and a higher DHEA-to-cortisol ratio is associated with fewer dissociative symptoms in healthy subjects during military survival training (Rasmusson et al., 2003; Mulchahey et al., 2001). There is evidence to suggest that testosterone promotes resilient behavior in males with MDD and PTSD, an observation congruent with the epidemiological data that women are at significantly higher risk than men to develop such disorders (Pope et al., 2003). One area of work that will significantly advance resilience research will be human brain imaging, as the elucidation of brain circuits involved in stress resilience is vital. Although this avenue of exploration still remains in its infancy in human studies, there already are some promising findings. Steffens et al. (2017), in the NBOLD study, have found significant differences in the Default Mode Network vmPFC/dlPFC that are thought to play a regulatory role in corticolimbic circuits mediating stress vulnerability (Steffens et al., 2017).

Treatment/implications/future Stress resilience neurobiology research, at its core, deals with the human evaluational response to verbal and nonverbal stimuli (aka “stressors”) in connection with their unique meanings to the “person.” Maladaptive evaluations result in abnormal stress responses, which lead to a cascade of negative consequences including poor coping skills, reduced tolerance for stressful stimuli, and higher risk of developing a psychiatric disorder (Hammen et al., 1985). As briefly discussed in this chapter, and further elaborated upon in others, genetic variants appear to interact with environmental variables to modulate how a human’s reaction to stress predisposes or protects against development of psychiatric disturbances. Studies suggest that the molecular mechanisms of childhood abuse and neglect as well as other forms of early-life adversity are potentially reversible in adulthood (Meaney and Szyf, 2005). That behavioral interventions have been shown to directly affect 5-HT neurotransmission, leading to changes in GR expression, which allow for effective termination of stress response bodes well for the field of stress resilience research. Not to be excluded, exercise training has also had consistently positive results suggesting utility in the area of stress resilience, specifically for those with clinical depression (Martinsen et al., 1985; Blumenthal et al., 1999; Singh et al., 2001). Exercise monotherapy for mild to moderate depression showed comparable rates of remission to the SSRI monotherapy group. Furthermore, during the follow-up period, those who exercised on their own had a 50% reduction in probability of relapse compared with those who did not continue exercise after study completion (Babyak et al., 2000; Salmon, 2001). In relation to stress responsivity, it is of interest that exercise-trained individuals showed attenuated HPA axis responses to mental stress (Luger et al., 1987) and that exercise has been shown to prevent stress-induced changes in gene expression of neurotrophic factors vital to hippocampal function (Russo-Neustadt et al., 2001).

References

191

The neurobiology of stress resilience will hopefully make possible the induction of natural mechanisms of resilience in vulnerable populations including victims of child abuse and neglect. One of the intrinsic and unavoidable challenges that researchers must navigate in the realm of stress resilience neurobiological research is that those who are able to maintain a high level of function and psychiatric stability despite exposure to trauma do not come to the attention of clinicians. As a result, other than some studies in niche populations (military personnel), the field remains challenged to discern resilient mechanisms from those that have been shown to have higher risk/vulnerability such as those who develop MDD/ PTSD in the context of childhood abuse and neglect. A plausible methodological approach that may better demonstrate the neurobiological mechanisms mediating resilient behavior would include a model that prospectively examines the impact of a stressor such as natural disaster with comparisons to nonetrauma-exposed individuals. Determining what factors contribute to psychiatric vulnerability and morbidity in these two groups in a long-term longitudinal paradigm is of interest. Decreasing the CRF response both centrally and peripherally to stress represents an important component of the therapeutic response in mood and anxiety disorders (Nemeroff and Vale, 2005). Alternatively, discovery of resilience biomarkers that translate into novel interventions that can alleviate the suffering of those afflicted by stress-related disorders and/or prevention is the ultimate goal.

Financial Disclosures The author(s) declared the following financial relationships over the past 3 years. Charles B. Nemeroff, MD, PhD, Research/Grants: National Institutes of Health (NIH), Stanley Foundation. Consulting: Xhale, Takeda, Mitsubishi Tanabe Pharma Development America, Taisho Pharmaceutical Inc., Navitor, Intracellular therapeutics, Bracket (Clintara), Gerson Lehrman Group (GLG) Healthcare & Biomedical Council, Sunovion Pharmaceuticals Inc., TC-MSO, Janssen Research & Development, LLC, Magstim, Inc.; Stockholder (or options): Xhale, Celgene, Seattle Genetics, Abbvie, OPKO Health, Inc., Bracket Intermediate Holding Corp., Network Life Sciences Inc.; Scientific Advisory Boards: American Foundation for Suicide Prevention (AFSP), Brain and Behavior Research Foundation (BBRF) (formerly named National Alliance for Research on Schizophrenia and Depression [NARSAD]), Xhale, Anxiety Disorders Association of America (ADAA), Skyland Trail, Bracket (Clintara), Laureate Institute for Brain Research, Inc.; Board of Directors: AFSP, Gratitude America, ADAA; Income sources or equity of US$10,000 or more: American Psychiatric Publishing, Xhale, Bracket (Clintara), CME Outfitters, Takeda; Patents: Method and devices for transdermal delivery of lithium (US 6,375,990B1) and method of assessing antidepressant drug therapy via transport inhibition of monoamine neurotransmitters by ex vivo assay (US 7,148,027B2).

References Agid, O., Shapira, B., Zislin, J., Ritsner, M., Hanin, B., Murad, H., et al., 1999. Environment and vulnerability to major psychiatric illness: a case control study of early parental loss in major depression, bipolar disorder and schizophrenia. Molecular Psychiatry 4 (2). Anda, R.F., Felitti, V.J., Bremner, J.D., Walker, J.D., Whitfield, C.H., Perry, B.D., et al., 2006. The enduring effects of abuse and related adverse experiences in childhood. European Archives of Psychiatry and Clinical Neuroscience 256 (3), 174e186. Andero, R., Brothers, S.P., Jovanovic, T., Chen, Y.T., Salah-Uddin, H., Cameron, M., et al., 2013. Amygdala-dependent fear is regulated by Oprl1 in mice and humans with PTSD. Science Translational Medicine 5 (188), 188ra73-ra73. Antoni, F.A., Palkovits, M., Makara, G.B., Linton, E.A., Lowry, P.J., Kiss, J.Z., 1983. Immunoreactive corticotropinreleasing hormone in the hypothalamoinfundibular tract. Neuroendocrinology 36 (6), 415e423. Arborelius, L., Owens, M.J., Plotsky, P.M., Nemeroff, C.B., 1999. The role of corticotropin-releasing factor in depression and anxiety disorders. Journal of Endocrinology 160 (1), 1e12.

192

13. Child abuse and neglect: stress responsivity and resilience

Babyak, M., Blumenthal, J.A., Herman, S., Khatri, P., Doraiswamy, M., Moore, K., et al., 2000. Exercise treatment for major depression: maintenance of therapeutic benefit at 10 months. Psychosomatic Medicine 62 (5), 633e638. Bale, T.L., Vale, W.W., 2004. CRF and CRF receptors: role in stress responsivity and other behaviors. Annual Review of Pharmacology and Toxicology 44, 525e557. Binder, E.B., Bradley, R.G., Liu, W., Epstein, M.P., Deveau, T.C., Mercer, K.B., et al., 2008. Association of FKBP5 polymorphisms and childhood abuse with risk of posttraumatic stress disorder symptoms in adults. Jama 299 (11), 1291e1305. Blumenthal, J.A., Babyak, M.A., Moore, K.A., Craighead, W.E., Herman, S., Khatri, P., et al., 1999. Effects of exercise training on older patients with major depression. Archives of Internal Medicine 159 (19), 2349e2356. Bonaccorso, S., Marino, V., Puzella, A., Pasquini, M., Biondi, M., Artini, M., et al., 2002. Increased depressive ratings in patients with hepatitis C receiving interferon-aebased immunotherapy are related to interferon-aeinduced changes in the serotonergic system. Journal of Clinical Psychopharmacology 22 (1), 86e90. Borland, L.M., Michael, A.C., 2004. Voltammetric study of the control of striatal dopamine release by glutamate. Journal of Neurochemistry 91 (1), 220e229. Bradley, C.C., Blakely, R.D., 1997. Alternative splicing of the human serotonin transporter gene. Journal of Neurochemistry 69 (4), 1356e1367. Bradley, R.G., Binder, E.B., Epstein, M.P., Tang, Y., Nair, H.P., Liu, W., et al., 2008. Influence of child abuse on adult depression: moderation by the corticotropin-releasing hormone receptor gene. Archives of General Psychiatry 65 (2), 190e200. Bremner, J.D., Southwick, S.M., Johnson, D.R., Yehuda, R., Charney, D.S., 1993. Childhood physical abuse and combat-related posttraumatic stress disorder in Vietnam veterans. American Journal of Psychiatry 150 (2), 235. Buss, C., Lord, C., Wadiwalla, M., Hellhammer, D.H., Lupien, S.J., Meaney, M.J., et al., 2007. Maternal care modulates the relationship between prenatal risk and hippocampal volume in women but not in men. Journal of Neuroscience 27 (10), 2592e2595. Capuron, L., Gumnick, J.F., Musselman, D.L., Lawson, D.H., Reemsnyder, A., Nemeroff, C.B., et al., 2002. Neurobehavioral Effects of Interferon-a in Cancer Patients: Phenomenology and Paroxetine Responsiveness of Symptom Dimensions. Carpenter, L.L., Tyrka, A.R., McDougle, C.J., Malison, R.T., Owens, M.J., Nemeroff, C.B., et al., 2004. Cerebrospinal fluid corticotropin-releasing factor and perceived early-life stress in depressed patients and healthy control subjects. Neuropsychopharmacology 29 (4), 777. Carpenter, L.L., Carvalho, J.P., Tyrka, A.R., Wier, L.M., Mello, A.F., Mello, M.F., et al., 2007. Decreased adrenocorticotropic hormone and cortisol responses to stress in healthy adults reporting significant childhood maltreatment. Biological Psychiatry 62 (10), 1080e1087. Carpenter, L.L., Tyrka, A.R., Ross, N.S., Khoury, L., Anderson, G.M., Price, L.H., 2009. Effect of childhood emotional abuse and age on cortisol responsivity in adulthood. Biological Psychiatry 66 (1), 69e75. Carpenter, L.L., Shattuck, T.T., Tyrka, A.R., Geracioti, T.D., Price, L.H., 2011. Effect of childhood physical abuse on cortisol stress response. Psychopharmacology 214 (1), 367e375. Caspi, A., McClay, J., Moffitt, T.E., Mill, J., Martin, J., Craig, I.W., et al., 2002. Role of genotype in the cycle of violence in maltreated children. Science 297 (5582), 851e854. Caspi, A., Sugden, K., Moffitt, T.E., Taylor, A., Craig, I.W., Harrington, H., et al., 2003. Influence of life stress on depression: moderation by a polymorphism in the 5-HTT gene. Science 301 (5631), 386e389. Chalmers, D.T., Lovenberg, T.W., Grigoriadis, D.E., Behan, D.P., De Souza, E.B., 1996. Corticotrophin-releasing factor receptors: from molecular biology to drug design. Trends in Pharmacological Sciences 17 (4), 166e172. Coelho, R., Viola, T.W., Walss-Bass, C., Brietzke, E., Grassi-Oliveira, R., 2014. Childhood maltreatment and inflammatory markers: a systematic review. Acta Psychiatrica Scandinavica 129 (3), 180e192. Costello, E.J., Pine, D.S., Hammen, C., March, J.S., Plotsky, P.M., Weissman, M.M., et al., 2002. Development and natural history of mood disorders. Biological Psychiatry 52 (6), 529e542. Danese, A., Pariante, C.M., Caspi, A., Taylor, A., Poulton, R., 2007. Childhood maltreatment predicts adult inflammation in a life-course study. Proceedings of the National Academy of Sciences 104 (4), 1319e1324. Danese, A., Caspi, A., Williams, B., Ambler, A., Sugden, K., Mika, J., et al., 2011. Biological embedding of stress through inflammation processes in childhood. Molecular Psychiatry 16 (3), 244.

References

193

Dannlowski, U., Stuhrmann, A., Beutelmann, V., Zwanzger, P., Lenzen, T., Grotegerd, D., et al., 2012. Limbic scars: long-term consequences of childhood maltreatment revealed by functional and structural magnetic resonance imaging. Biological Psychiatry 71 (4), 286e293. De Bellis, M.D., Thomas, L.A., 2003. Biologic findings of post-traumatic stress disorder and child maltreatment. Current Psychiatry Reports 5 (2), 108e117. De Kloet, E.R., Joëls, M., Holsboer, F., 2005. Stress and the brain: from adaptation to disease. Nature Reviews Neuroscience 6 (6), 463. DeRijk, R., de Kloet, E.R., 2005. Corticosteroid receptor genetic polymorphisms and stress responsivity. Endocrine 28 (3), 263e269. Ellis, B.J., Essex, M.J., Boyce, W.T., 2005. Biological sensitivity to context: II. Empirical explorations of an evolutionaryedevelopmental theory. Development and Psychopathology 17 (2), 303e328. Elzinga, B.M., Roelofs, K., Tollenaar, M.S., Bakvis, P., van Pelt, J., Spinhoven, P., 2008. Diminished cortisol responses to psychosocial stress associated with lifetime adverse events: a study among healthy young subjects. Psychoneuroendocrinology 33 (2), 227e237. El-Sheikh, M., 2005. The role of emotional responses and physiological reactivity in the marital conflictechild functioning link. Journal of Child Psychology and Psychiatry 46 (11), 1191e1199. Felger, J.C., Li, L., Marvar, P.J., Woolwine, B.J., Harrison, D.G., Raison, C.L., et al., 2013. Tyrosine metabolism during interferon-alpha administration: association with fatigue and CSF dopamine concentrations. Brain, Behavior, and Immunity 31, 153e160. Fernando, S.C., Beblo, T., Schlosser, N., Terfehr, K., Otte, C., Löwe, B., et al., 2012. Associations of childhood trauma with hypothalamic-pituitary-adrenal function in borderline personality disorder and major depression. Psychoneuroendocrinology 37 (10), 1659e1668. Frodl, T., Reinhold, E., Koutsouleris, N., Reiser, M., Meisenzahl, E.M., 2010. Interaction of childhood stress with hippocampus and prefrontal cortex volume reduction in major depression. Journal of Psychiatric Research 44 (13), 799e807. Grabe, H.J., Schwahn, C., Mahler, J., Schulz, A., Spitzer, C., Fenske, K., et al., 2012. Moderation of adult depression by the serotonin transporter promoter variant (5-HTTLPR), childhood abuse and adult traumatic events in a general population sample. American Journal of Medical Genetics Part B: Neuropsychiatric Genetics 159 (3), 298e309. Gunnar, M., Quevedo, K., 2007. The neurobiology of stress and development. Annual Review of Psychology 58, 145e173. Gutman, D.A., Nemeroff, C.B., 2002. Neurobiology of early life stress: rodent studies. Seminars in Clinical Neuropsychiatry 7, 89e95. Review. PubMed PMID: 11953932. Hammen, C., Marks, T., Mayol, A., DeMayo, R., 1985. Depressive self-schemas, life stress, and vulnerability to depression. Journal of Abnormal Psychology 94 (3), 308. Harrison, N.A., Brydon, L., Walker, C., Gray, M.A., Steptoe, A., Critchley, H.D., 2009. Inflammation causes mood changes through alterations in subgenual cingulate activity and mesolimbic connectivity. Biological Psychiatry 66 (5), 407e414. Health USDo, Human S, 2015. Child Maltreatment. Heim, C., Binder, E.B., 2012. Current research trends in early life stress and depression: review of human studies on sensitive periods, geneeenvironment interactions, and epigenetics. Experimental Neurology 233 (1), 102e111. Heim, C., Nemeroff, C.B., 2001. The role of childhood trauma in the neurobiology of mood and anxiety disorders: preclinical and clinical studies. Biological Psychiatry 49 (12), 1023e1039. Heim, C., Newport, D.J., Heit, S., Graham, Y.P., Wilcox, M., Bonsall, R., et al., 2000. Pituitary-adrenal and autonomic responses to stress in women after sexual and physical abuse in childhood. Jama 284 (5), 592e597. Heim, C., Ehlert, U., Hellhammer, D.H., 2000. The potential role of hypocortisolism in the pathophysiology of stress-related bodily disorders. Psychoneuroendocrinology 25 (1), 1e35. Heim, C., Newport, D.J., Wagner, D., Wilcox, M.M., Miller, A.H., Nemeroff, C.B., 2002. The role of early adverse experience and adulthood stress in the prediction of neuroendocrine stress reactivity in women: a multiple regression analysis. Depression and Anxiety 15 (3), 117e125. Heim, C., Newport, D.J., Bonsall, R., Miller, A.H., Nemeroff, C.B., 2003. Altered pituitary-adrenal axis responses to provocative challenge tests in adult survivors of childhood abuse. Focus 1 (3), 282e289.

194

13. Child abuse and neglect: stress responsivity and resilience

Heim, C., Mletzko, T., Purselle, D., Musselman, D.L., Nemeroff, C.B., 2008. The dexamethasone/ corticotropin-releasing factor test in men with major depression: role of childhood trauma. Biological Psychiatry 63 (4), 398e405. Heim, C.M., Mayberg, H.S., Mletzko, T., Nemeroff, C.B., Pruessner, J.C., 2013. Decreased cortical representation of genital somatosensory field after childhood sexual abuse. American Journal of Psychiatry 170 (6), 616e623. Herman, J.P., Cullinan, W.E., 1997. Neurocircuitry of stress: central control of the hypothalamoepituitarye adrenocortical axis. Trends in Neurosciences 20 (2), 78e84. Herman, J.P., Cullinan, W.E., Ziegler, D.R., Tasker, J.G., 2002. Role of the paraventricular nucleus microenvironment in stress integration. European Journal of Neuroscience 16 (3), 381e385. Irwin, M.R., Miller, A.H., 2007. Depressive disorders and immunity: 20 years of progress and discovery. Brain, Behavior, and Immunity 21 (4), 374e383. Jacobson, L., Sapolsky, R., 1991. The role of the hippocampus in feedback regulation of the hypothalamic-pituitaryadrenocortical axis. Endocrine Reviews 12 (2), 118e134. Karg, K., Burmeister, M., Shedden, K., Sen, S., 2011. The serotonin transporter promoter variant (5-HTTLPR), stress, and depression meta-analysis revisited: evidence of genetic moderation. Archives of General Psychiatry 68 (5), 444e454. Kaufman, J., Yang, B.-Z., Douglas-Palumberi, H., Houshyar, S., Lipschitz, D., Krystal, J.H., et al., 2004. Social supports and serotonin transporter gene moderate depression in maltreated children. Proceedings of the National Academy of Sciences of the United States of America 101 (49), 17316e17321. Kaufman, J., Yang, B.-Z., Douglas-Palumberi, H., Grasso, D., Lipschitz, D., Houshyar, S., et al., 2006. Brain-derived neurotrophic factore5-HTTLPR gene interactions and environmental modifiers of depression in children. Biological Psychiatry 59 (8), 673e680. Kirschbaum, C., Pirke, K.-M., Hellhammer, D.H., 1993. The ‘Trier Social Stress Test’ea tool for investigating psychobiological stress responses in a laboratory setting. Neuropsychobiology 28 (1e2), 76e81. Krishnan, V., Han, M.-H., Graham, D.L., Berton, O., Renthal, W., Russo, S.J., et al., 2007. Molecular adaptations underlying susceptibility and resistance to social defeat in brain reward regions. Cell 131 (2), 391e404. Krügel, U., Fischer, J., Radicke, S., Sack, U., Himmerich, H., 2013. Antidepressant effects of TNF-a blockade in an animal model of depression. Journal of Psychiatric Research 47 (5), 611e616. Laucht, M., Treutlein, J., Blomeyer, D., Buchmann, A.F., Schmidt, M.H., Esser, G., et al., 2013. Interactive effects of corticotropin-releasing hormone receptor 1 gene and childhood adversity on depressive symptoms in young adults: findings from a longitudinal study. European Neuropsychopharmacology 23 (5), 358e367. Luger, A., Deuster, P.A., Kyle, S.B., Gallucci, W.T., Montgomery, L.C., Gold, P.W., et al., 1987. Acute hypothalamice pituitaryeadrenal responses to the stress of treadmill exercise. New England Journal of Medicine 316 (21), 1309e1315. Maes, M., 1995. Evidence for an immune response in major depression: a review and hypothesis. Progress in NeuroPsychopharmacology and Biological Psychiatry 19 (1), 11e38. Maes, M., Meltzer, H.Y., Scharpè, S., Bosmans, E., Suy, E., De Meester, I., et al., 1993. Relationships between lower plasma L-tryptophan levels and immune-inflammatory variables in depression. Psychiatry Research 49 (2), 151e165. Maes, M., Yirmyia, R., Noraberg, J., Brene, S., Hibbeln, J., Perini, G., et al., 2009. The inflammatory & neurodegenerative (I&ND) hypothesis of depression: leads for future research and new drug developments in depression. Metabolic Brain Disease 24 (1), 27e53. Maes, M., Leonard, B.E., Myint, A.M., Kubera, M., Verkerk, R., 2011. The new ‘5-HT’hypothesis of depression: cell-mediated immune activation induces indoleamine 2, 3-dioxygenase, which leads to lower plasma tryptophan and an increased synthesis of detrimental tryptophan catabolites (TRYCATs), both of which contribute to the onset of depression. Progress in Neuro-Psychopharmacology and Biological Psychiatry 35 (3), 702e721. Martinsen, E.W., Medhus, A., Sandvik, L., 1985. Effects of aerobic exercise on depression: a controlled study. British Medical Journal 291 (6488), 109. McEwen, B.S., 1998. Stress, adaptation, and disease: allostasis and allostatic load. Annals of the New York Academy of Sciences 840 (1), 33e44. McEwen, B.S., 2004. Protection and damage from acute and chronic stress: allostasis and allostatic overload and relevance to the pathophysiology of psychiatric disorders. Annals of the New York Academy of Sciences 1032 (1), 1e7.

References

195

McLaughlin, K.A., Sheridan, M.A., Alves, S., Mendes, W.B., 2014. Child maltreatment and autonomic nervous system reactivity: identifying dysregulated stress reactivity patterns using the biopsychosocial model of challenge and threat. Psychosomatic Medicine 76 (7), 538. Meaney, M.J., Szyf, M., 2005. Environmental programming of stress responses through DNA methylation: life at the interface between a dynamic environment and a fixed genome. Dialogues in Clinical Neuroscience 7 (2), 103. Molnar, B.E., Buka, S.L., Kessler, R.C., 2001. Child sexual abuse and subsequent psychopathology: results from the National Comorbidity Survey. American Journal of Public Health 91 (5), 753. Monroe, S.M., Simons, A.D., 1991. Diathesis-stress theories in the context of life stress research: implications for the depressive disorders. Psychological Bulletin 110 (3), 406. Mulchahey, J.J., Ekhator, N.N., Zhang, H., Kasckow, J.W., Baker, D.G., Geracioti, T.D., 2001. Cerebrospinal fluid and plasma testosterone levels in post-traumatic stress disorder and tobacco dependence. Psychoneuroendocrinology 26 (3), 273e285. Munafo, M.R., Clark, T., Flint, J., 2005. Does measurement instrument moderate the association between the serotonin transporter gene and anxiety-related personality traits? A meta-analysis. Molecular Psychiatry 10 (4), 415. Nanni, V., Uher, R., Danese, A., 2012. Childhood maltreatment predicts unfavorable course of illness and treatment outcome in depression: a meta-analysis. American Journal of Psychiatry 169 (2), 141e151. Nemeroff, C.B., 1996. The corticotropin-releasing factor (CRF) hypothesis of depression: new findings and new directions. Molecular Psychiatry 1 (4), 336e342. Nemeroff, C.B., Vale, W.W., 2005. The neurobiology of depression: inroads to treatment and new drug discovery. Journal of Clinical Psychiatry 66, 5e13. Nestler, E.J., Barrot, M., DiLeone, R.J., Eisch, A.J., Gold, S.J., Monteggia, L.M., 2002. Neurobiology of depression. Neuron 34 (1), 13e25. Neurauter, G., Schrocksnadel, K., Scholl-Burgi, S., Sperner-Unterweger, B., Schubert, C., Ledochowski, M., et al., 2008. Chronic immune stimulation correlates with reduced phenylalanine turnover. Current Drug Metabolism 9 (7), 622e627. Newport, D.J., Heim, C., Bonsall, R., Miller, A.H., Nemeroff, C.B., 2004. Pituitary-adrenal responses to standard and low-dose dexamethasone suppression tests in adult survivors of child abuse. Biological Psychiatry 55 (1), 10e20. Oosterman, M., De Schipper, J.C., Fisher, P., Dozier, M., Schuengel, C., 2010. Autonomic reactivity in relation to attachment and early adversity among foster children. Development and Psychopathology 22 (1), 109e118. Opel, N., Redlich, R., Zwanzger, P., Grotegerd, D., Arolt, V., Heindel, W., et al., 2014. Hippocampal atrophy in major depression: a function of childhood maltreatment rather than diagnosis? Neuropsychopharmacology 39 (12), 2723. Pace, T.W.W., Wingenfeld, K., Schmidt, I., Meinlschmidt, G., Hellhammer, D.H., Heim, C.M., 2012. Increased peripheral NF-kB pathway activity in women with childhood abuse-related posttraumatic stress disorder. Brain, Behavior, and Immunity 26 (1), 13e17. Pope Jr., H.G., Cohane, G.H., Kanayama, G., Siegel, A.J., Hudson, J.I., 2003. Testosterone gel supplementation for men with refractory depression: a randomized, placebo-controlled trial. American Journal of Psychiatry 160 (1), 105e111. Qin, L., Wu, X., Block, M.L., Liu, Y., Breese, G.R., Hong, J.S., et al., 2007. Systemic LPS causes chronic neuroinflammation and progressive neurodegeneration. Glia 55 (5), 453e462. Raison, C.L., Miller, A.H., 2003. When not enough is too much: the role of insufficient glucocorticoid signaling in the pathophysiology of stress-related disorders. American Journal of Psychiatry 160 (9), 1554e1565. Raison, C.L., Capuron, L., Miller, A.H., 2006. Cytokines sing the blues: inflammation and the pathogenesis of depression. Trends in Immunology 27 (1), 24e31. Raison, C.L., Dantzer, R., Kelley, K.W., Lawson, M.A., Woolwine, B.J., Vogt, G., et al., 2010. CSF concentrations of brain tryptophan and kynurenines during immune stimulation with IFN-a: relationship to CNS immune responses and depression. Molecular Psychiatry 15 (4), 393e403. Rasmusson, A.M., Vythilingam, M., Morgan, C.A., 2003. The neuroendocrinology of posttraumatic stress disorder: new directions. CNS Spectrums 8 (9), 651e667. Reichenberg, A., Yirmiya, R., Schuld, A., Kraus, T., Haack, M., Morag, A., et al., 2001. Cytokine-associated emotional and cognitive disturbances in humans. Archives of General Psychiatry 58 (5), 445e452. Rellini, A.H., Meston, C.M., 2006. Psychophysiological sexual arousal in women with a history of child sexual abuse. Journal of Sex & Marital Therapy 32 (1), 5e22.

196

13. Child abuse and neglect: stress responsivity and resilience

Ressler, K.J., Bradley, B., Mercer, K.B., Deveau, T.C., Smith, A.K., Gillespie, C.F., et al., 2010. Polymorphisms in CRHR1 and the serotonin transporter loci: gene gene environment interactions on depressive symptoms. American Journal of Medical Genetics Part B: Neuropsychiatric Genetics 153 (3), 812e824. Russo, S.J., Murrough, J.W., Han, M.-H., Charney, D.S., Nestler, E.J., 2012. Neurobiology of resilience. Nature Neuroscience 15 (11), 1475e1484. Russo-Neustadt, A., Ha, T., Ramirez, R., Kesslak, J.P., 2001. Physical activityeantidepressant treatment combination: impact on brain-derived neurotrophic factor and behavior in an animal model. Behavioural Brain Research 120 (1), 87e95. Salmon, P., 2001. Effects of physical exercise on anxiety, depression, and sensitivity to stress: a unifying theory. Clinical Psychology Review 21 (1), 33e61. Sanchez, M.M., Young, L.J., Plotsky, P.M., Insel, T.R., 1999. Autoradiographic and in situ hybridization localization of corticotropin-releasing factor 1 and 2 receptors in nonhuman primate brain. The Journal of Comparative Neurology 408 (3), 365e377. Schoedl, A.F., Costa, M.C.P., Mari, J.J., Mello, M.F., Tyrka, A.R., Carpenter, L.L., et al., 2010. The clinical correlates of reported childhood sexual abuse: an association between age at trauma onset and severity of depression and PTSD in adults. Journal of Child Sexual Abuse 19 (2), 156e170. Shekhar, A., Truitt, W., Rainnie, D., Sajdyk, T., 2005. Role of stress, corticotrophin releasing factor (CRF) and amygdala plasticity in chronic anxiety. Stress: The International Journal on the Biology of Stress 8 (4), 209e219. Singh, N.A., Clements, K.M., Singh, M.A.F., 2001. The efficacy of exercise as a long-term antidepressant in elderly subjects: a randomized, controlled trial. The Journals of Gerontology Series A: Biological Sciences and Medical Sciences 56 (8), M497eM504. Steckler, T., Holsboer, F., 1999. Corticotropin-releasing hormone receptor subtypes and emotion. Biological Psychiatry 46 (11), 1480e1508. Steffens, D.C., Wang, L., Manning, K.J., Pearlson, G.D., 2017. Negative affectivity, aging, and depression: results from the neurobiology of late-life depression (NBOLD) study. The American Journal of Geriatric Psychiatry 25 (10), 1135e1149. Stein, M.B., Simmons, A.N., Feinstein, J.S., Paulus, M.P., 2007. Increased amygdala and insula activation during emotion processing in anxiety-prone subjects. American Journal of Psychiatry 164 (2), 318e327. Steiner, J., Walter, M., Gos, T., Guillemin, G.J., Bernstein, H.-G., Sarnyai, Z., et al., 2011. Severe depression is associated with increased microglial quinolinic acid in subregions of the anterior cingulate gyrus: evidence for an immune-modulated glutamatergic neurotransmission? Journal of Neuroinflammation 8 (1), 94. Swanson, L.W., Sawchenko, P.E., 1983. Hypothalamic integration: organization of the paraventricular and supraoptic nuclei. Annual Review of Neuroscience 6 (1), 269e324. Swiergiel, A.H., Takahashi, L.K., Kalin, N.H., 1993. Attenuation of stress-induced behavior by antagonism of corticotropin-releasing factor receptors in the central amygdala in the rat. Brain Research 623 (2), 229e234. Teicher, M.H., Anderson, C.M., Polcari, A., 2012. Childhood maltreatment is associated with reduced volume in the hippocampal subfields CA3, dentate gyrus, and subiculum. Proceedings of the National Academy of Sciences 109 (9), E563eE572. Urani, A., Chourbaji, S., Gass, P., 2005. Mutant mouse models of depression: candidate genes and current mouse lines. Neuroscience & Biobehavioral Reviews 29 (4), 805e828. Vythilingam, M., Heim, C., Newport, J., Miller, A.H., Anderson, E., Bronen, R., et al., 2002. Childhood trauma associated with smaller hippocampal volume in women with major depression. American Journal of Psychiatry 159 (12), 2072e2080. Weiss, E.L., Longhurst, J.G., Mazure, C.M., 1999. Childhood sexual abuse as a risk factor for depression in women: psychosocial and neurobiological correlates. American Journal of Psychiatry 156 (6), 816e828. Yehuda, R., 2001. Biology of posttraumatic stress disorder. Journal of Clinical Psychiatry 62, 41e46. Yehuda, R., 2006. Advances in understanding neuroendocrine alterations in PTSD and their therapeutic implications. Annals of the New York Academy of Sciences 1071 (1), 137e166. Yehuda, R., Giller, E.L., Southwick, S.M., Lowy, M.T., Mason, J.W., 1991. Hypothalamic-pituitary-adrenal dysfunction in posttraumatic stress disorder. Biological Psychiatry 30 (10), 1031e1048. Zhu, C.-B., Lindler, K.M., Owens, A.W., Daws, L.C., Blakely, R.D., Hewlett, W.A., 2010. Interleukin-1 receptor activation by systemic lipopolysaccharide induces behavioral despair linked to MAPK regulation of CNS serotonin transporters. Neuropsychopharmacology 35 (13), 2510e2520.

C H A P T E R

14

How genes and environment interact to shape risk and resilience to stress-related psychiatric disorders 1

Lilia Papst1, Elisabeth B. Binder1, 2

Department of Translational Research in Psychiatry, Max Planck Institute of Psychiatry, Munich, Germany; 2Department of Psychiatry and Behavioral Sciences and Department of Psychology, Emory University School of Medicine, Atlanta, GA, United States

Introduction Mental disorders are multicausal and diagnostically overlapping and typically take a chronic course over prolonged periods of time, with recurring symptoms often triggered through stressful experiences. Although the identification of definitive causal factors has proven difficult, the diathesis-stress model has garnered widespread acceptance in the field with its general assumption that psychiatric disorders are caused by a combination of genetic and environmental factors. The contribution of genetic variation to pathogenesis differs between diagnoses but is mainly polygenic with many genes contributing with small effect sizes as opposed to single gene large effects. Twin and family studies and now also genome-wide association studies (GWAS) report the genetic contribution to psychiatric disorders to range from about 80% for autism to about 30%e40% for major depressive disorder (MDD) (Sullivan et al., 2012). Environmental factors, especially adverse life events, can both trigger or exacerbate disease course. Among these, childhood adversity has been most consistently associated with increased risk for a range of psychiatric symptoms. In a study including data from 21 countries, childhood adversities, in particular when associated with maladaptive family functioning, strongly associated with psychiatric disorders, with little specificity across disorders and explaining up to 30% of disease variance across diagnoses (Kessler et al., 2010). Given that exposure to stressful life experiences can often not be avoided, building resilience constitutes an integral part of preventive and therapeutic efforts. Understanding

Stress Resilience https://doi.org/10.1016/B978-0-12-813983-7.00014-8

197

Copyright © 2020 Elsevier Inc. All rights reserved.

198

14. How genes and environment interact to shape risk and resilience to stress-related psychiatric disorders

resilience-promoting factors as well as interindividual differences in resilience may help to optimize these efforts. Resilience-promoting factors include caring and supportive relationships, communication and problem-solving skills, and the ability to manage strong feelings and impulses (American Psychological Association, 2018). The mechanisms underlying resilience may be primed early on by the interplay of environmental factors, including the quality of caregiving and the degree of adversity, and genetic factors that impact on the regulation of the stress response, which in turn may influence the development of brain circuits relevant for emotion regulation (Blair and Raver, 2012). In this chapter, we will discuss how the interplay of genetic and environmental factors shapes the developing organism to ease or perturb resilience-promoting qualities through their influence on intermediate phenotypes. The chapter structures the findings by giving specific examples for such mechanisms along developmental periods. A focus will be on genetic polymorphisms in select candidate genes that moderate the impact of adversity to predict adult psychiatric disorders (see Halldorsdottir and Binder, 2017 for review). We will highlight examples of mechanisms of how these genetic polymorphisms influence specific biological mechanisms throughout development. These include but are not limited to genes involved in regulating the stress hormone system (FKBP5 and CRFR1), the oxytocin system (oxytocin receptor gene [OXTR] and the gene encoding the oxytocin peptide [OXT]) as well as the monoamine system (SLC6A4 and COMT).

Prenatal development The human embryonic stage is a highly dynamic phase of brain development, governed by a complex interplay of intrinsic and extrinsic cellular signaling. Extrinsic signaling events may hereby involve external perturbances, such as increased glucocorticoid levels, which can significantly alter brain development trajectories (O’Donnell and Meaney, 2016). Although the unborn child is usually protected from high glucocorticoid levels by a barrier of placental 11b-hydroxysteroid dehydrogenase type 2, an enzyme that degrades cortisol to inactive metabolites, this mechanism can be compromised as a result of maternal stress (Mairesse et al., 2007) or depression (Seth et al., 2015). Genetic factors act as moderators of these processes by altering the impact of glucocorticoids at the level of the placenta or the developing embryo. Schizophrenia, for instance, has long been known to have a strong genetic background, while obstetric complications have been shown to be a strong environmental risk factor for this disorder. A recent study revealed that the interaction of polygenic risk scores derived from large GWAS for schizophrenia with obstetric complications best explained variance in disease risk (Ursini et al., 2018). In fact, genes within this polygenic score were enriched for transcripts with strong expression in the placenta and also differentially expressed in placentae from complicated in comparison with normal pregnancies. This suggests that genetic polymorphisms predisposing to schizophrenia may in part mediate this risk by making the placenta more vulnerable during obstetric complications. In addition to obstetric complications, maternal stress and adversity during pregnancy have also been shown to increase risk for later psychiatric disorders, especially mood and anxiety disorders (Brannigan et al., 2018). Such effects may be mediated by alternating the trajectory of brain development. In interaction with prenatal adversity, a polygenic risk score for MDD was associated with reduced frontocortical gray matter, larger amygdala volumes,

Infancy

199

and altered hippocampal volume and shape at birth (Qiu et al., 2017). The same brain regions were affected in neonates by variation in candidate genes FKBP5, a gene influencing glucocorticoid receptor sensitivity, and the catechol-O-methyltransferase COMT, essential for catecholamine metabolism, in interaction with maternal mental health. A set of 19 FKBP5 SNPs predicted larger hippocampal volume in newborns (Wang et al., 2018), whereas a protein coding variation in COMT rs4680 resulted in reduced ventrolateral prefrontal cortex thickness in homozygotes for the lower activity methionine allele (Qiu et al., 2015). Such altered morphology in the frontal cortex and amygdala may contribute to behavioral problems observed in youth exposed to prenatal stress, who were more likely to exhibit negative emotionality, internalizing and externalizing behavior, disturbed motor development, and attentional and cognitive deficits (van den Bergh et al., 2017). Prenatal stress together with genetic risk factors may thus predispose children to a reduced capacity for frontally mediated emotion regulation very early in life. Additionally, both delay in motor functions and deficiencies in attention and general cognitive functions observed in exposed children may lead to early experiences of failure or inadequacy in the school setting. Altogether, these experiences are likely to amount to feelings of helplessness and low self-esteem from an early age. Understanding these mechanisms of prenatal risk may offer an opportunity for early intervention, using targeted environmental and behavioral support in the postnatal environment.

Infancy The first years of postnatal life are associated with sustained high plasticity, signifying both ongoing vulnerability and the chance for early interventions following prenatal adversity. Although gene expression in the brain of transcripts associated with cell proliferation decreases, genes related to myelination, synapse, and dendrite development continue to rise in expression levels (Kang et al., 2011). Meanwhile, social development is characterized by the formation of attachment to a primary caregiver (Bowlby, 1954). In fact, positive parental attention and physical contact were shown to attenuate or even eliminate the negative influence of prenatal stress (Sharp et al., 2012, 2015), although its effects persisted in most studies where these behaviors were not explicitly encouraged (Velders et al., 2011). An interesting candidate gene for G  E interactions in this context is the oxytocin receptor gene (OXTR) and the gene encoding the oxytocin peptide (OXT). Oxytocin is a hormone that biologically serves a function in cervix dilation before birth and stimulating lactation during breastfeeding but has been implicated in a much wider range of maternal and nurturing behaviors, emotional bonding, and social interactions. Genetic variation in OXT and OXTR may contribute to differences in the capacity for social interactions, both on the parental and on the offspring’s side. For instance, a gain-of-function genetic polymorphism in rhesus macaque OXTR was shown to partially protect from the negative behavioral consequences of early maternal deprivation in infants (Baker et al., 2017). In humans, genetic variation has also been shown to alter infant attachment, with OXTR SNP rs2254298 A allele carrier infants being more often securely attached than G homozygotes (Chen et al., 2011a). These polymorphisms in OXTR may therefore help infants to establish close relationships and may indirectly determine the extent of care and affection they are likely to receive from caregivers.

200

14. How genes and environment interact to shape risk and resilience to stress-related psychiatric disorders

OXTR polymorphisms were also shown to influence maternal behavior, with mothers with a specific OXTR genotype combination and adult separation anxiety showing significantly reduced levels of maternal sensitivity during free play with their infant (Mehta et al., 2016). Genetic variation in the gene encoding the OXT peptide was shown to interact with a mother’s own early parenting experience to predict the quality of her mothering (MilevaSeitz et al., 2013). The literature thus depicts a complex interaction of maternal and infant genetic variation in the oxytocin system to predict the quality of maternal care as well as the susceptibility of infants to poor care. As mentioned previously, caring and supportive relationships as well as good communication skills are important resilience factors with regard to mental health. Carriers of certain polymorphisms in OXT or OXTR may therefore be more resilient to adverse experiences. More importantly, however, physical contact and positive parental attention may be a sufficient means to counteract the effects of early stress and by extension the likelihood of offspring psychopathology. In fact, both in animal models and humans, it has been shown that close physical contact early in life can reduce later consequences of early adversity (Barrett et al., 2015; Hunziker and Barr, 1986).

Childhood According to the WHO World Mental Health Survey, childhood may be a uniquely sensitive phase for psychological development, as childhood stress accounts for about 30% of the variance of all psychiatric disorders (Kessler et al., 2010). While newborn infants predominantly rely on external stress regulation by means of their caretaker, children increasingly develop their own and more diverse strategies to cope with distress. Maladaptive coping in children is often broadly grouped into internalizing strategies, including rumination, worrying, and social withdrawal, and externalizing strategies, such as verbal or physical aggression. These behavioral coping strategies have been shown to associate with genetic polymorphisms in genes within the monoamine as well as stress-hormone system, thus providing a mechanism for genetic influence on later psychiatric risk. Both internalizing and externalizing symptoms were more common in children carrying a lower function (short) allele of a promoter polymorphism of the serotonin transporter gene SLC6A4 who were exposed to harsh parenting and traumatic events. Exposed children carrying the opposite allele were less affected and showed beneficial distraction coping strategies (Cline et al., 2015). Beneficial coping strategies in children were furthermore reported for carriers of alleles of other genes associated with adult mental health following childhood adversity. These included CRFR1, a gene encoding for the corticotrophin-releasing factor receptor important in the regulation of stress signaling (Cicchetti and Rogosch, 2012; Cline et al., 2015), OXTR (Cicchetti and Rogosch, 2012; Apter-Levy et al., 2013; Baribeau et al., 2017) or COMT (Hygen et al., 2015). These findings support that specific genotypes may increase resilience by promoting beneficial coping strategies when exposed to adversity. During childhood, the brain undergoes a phase of maturation marked by progressive loss of gray matter, starting in limbic brain areas and concluding with the frontal cortex (Gogtay et al., 2004). Interestingly, stressful experiences early in life may promote these structural changes. Childhood adversity has been associated with reduced gray matter volumes in

Adolescence

201

the frontal cortex, anterior cingulate cortex (ACC), insula, and hippocampus (Dannlowski et al., 2012), and these effects were more pronounced in individuals carrying specific risk alleles in FKBP5 (Grabe et al., 2016) or the SLC6A4 short allele (Frodl et al., 2010). Decreases in gray matter volume have been associated with processes of integration and specialization during this period. Exposure to early-life stress may therefore strengthen stimulus-response contingencies between a stressor and internalizing or externalizing behaviors that are adaptive at this time. However, by reducing developmental plasticity, learning of alternative coping strategies later in life could be reduced in exposed children. In fact, a multilocus score of genetic variations predicting higher HPA axis activity in FKBP5, CRFR1, and the mineralocorticoid receptor (NR3C2) was shown to associate with high amygdala reactivity to threat in young adults exposed to early-life stress. Since the amygdala is involved in the encoding of emotional memories, including negative ones, this process seems to be enhanced in individuals with experiences of early-life stress ranking high on this biologically informed multilocus profile score (Di Iorio et al., 2017). The hypothesis that early-life stress may promote reduction in gray matter is further corroborated by a genome-wide molecular study that integrated blood transcriptome data from humans exposed to childhood trauma with hippocampal microRNA and mRNA data from a prenatal stress rat model. Using this convergent transspecies approach, the authors identified FoxO1, a gene promoting neural cell death and TGFb1, which controls SMAD signaling, important for brain homeostasis and neural proliferation and differentiation, as dysregulated by early adversity. These two genes are representative of major processes in child brain maturationdapoptosis and neuronal differentiation. The relevance of FoxO1 could be further corroborated using a gene  environment interaction analysis in a large human cohort, where six polymorphisms in FoxO1 interacted with early-life stress to predict adult depressive symptoms (Cattaneo et al., 2018). With regard to possibilities of intervention, coping strategies might be suitable targets in spite of preexisting genetic preferences. For instance, children may feel encouraged to employ more adaptive stress regulation strategies when observing positive consequences in social role models (Bandura et al., 1963). An early establishment of such competencies, especially in children at genetic risk, could prevent trajectories of risk.

Adolescence Although early-life experiences significantly shape the risk to psychopathology, a remarkable number of psychiatric disorders do not have their onset until adolescence (Kessler et al., 2005). This may be due to both an increased exposure to social stressors and unique developmental features of adolescence. For instance, sex-dependent differences in susceptibility to individual diagnoses are not reported at younger ages (Angold et al., 1999), suggesting an important role of gonadal steroids, possibly in interaction with glucocorticoids. Genetic variation in the estrogen receptor alpha gene (ESR1) are, for example, associated with the likelihood to suffer from premenstrual dysphoric disorder (Huo et al., 2007) and interact with traumatic life events to predict PTSD symptomatology (Feng et al., 2018). In addition to hormonal changes, adolescence is characterized by heightened reward sensitivity, sensation-seeking, and immature impulse control (Steinberg and Chein, 2015),

202

14. How genes and environment interact to shape risk and resilience to stress-related psychiatric disorders

likely because of the fact that frontal cortex regions associated with cognitive control mature last, after limbic brain areas are already fully myelinated (Pfefferbaum et al., 1994). Differences in frontolimbic structures are therefore presumed at the core of problems in adolescence associated with the reward and emotion regulation systems. Females carrying genotypes associated with an extreme reduction in COMT (22q11 deletion and carrying the low activity rs4680 methionine allele), for example, exhibited excessive cortical thinning and deficits in executive function, but only after puberty (Sannino et al., 2017). Such complex interactions between brain maturation, stress, and genetic risk have been elegantly described by Niwa et al. where glucocorticoids link adolescent stressors to epigenetic control in neurons. Specifically, isolation stress in adolescence led to hypermethylation of the tyrosine hydroxylase promoter in mesolimbic dopamine projections, but only in animals carrying a dominantnegative DISC1 mutation, associated with neuropsychiatric risk in humans. This led to increased dopamine levels, heightened dopamine receptor expression in the frontal cortex, and behavioral impairments, which were successfully prevented by the application of a GR antagonist (Niwa et al., 2013). Well-established coping strategies to regulate feelings and impulses may help to contain some of these stress-induced biological alterations. However, especially, adolescents with specific genetic risk variants already described above, such as in FKBP5, OXTR, and the serotonin transporter may be less able to employ such strategies. For example, adolescents carrying the FKBP5 risk alleles growing up in households characterized by lower socioeconomic status and family adversity were more prone to employ negative strategies such as rumination or catastrophizing and develop symptoms of anxiety and depression (Halldorsdottir et al., 2017). Carriers of the 5-HTTLPR short allele had a higher risk of emotional dysregulation, lowered positive affect, and aggressive behavior when attachment was low or if they were exposed to unsupportive parenting (Zimmermann and Spangler, 2016; Hankin et al., 2011). However, securely attached OXTR rs2254298 A allele carriers also exhibited more depressive symptoms if their mothers experienced depression (Thompson et al., 2011), stressing the context specificity of adaptive genotypes. The latter finding stresses the importance of targeting intervention in adolescence not only to the individual but also to the whole family. Adolescents were shown to profit from interventions promoting supportive parental behavior as well as skills training, resulting in improvements on the behavioral level, which were accompanied by morphological changes in brain structure (Brody et al., 2009).

Adulthood All mechanisms discussed in the previous sections will impact on resilience in adulthood. Although gross morphological brain maturation processes are complete at this point, some adult neurogenic niches remain, such as in the subgranular zone of the hippocampal dentate gyrus (Eriksson et al., 1998). Being associated with the encoding of memory and learning of new behavior in adulthood (Deng et al., 2010), it may provide a possible gateway for both pharmacological and psychotherapeutic interventions (Alboni et al., 2017). In addition, synaptic plasticity allows learning and memory into old age. Among the most stressful life events in adulthood are the death of a child or other close family member, divorce, personal

Conclusions

203

illness, and unemployment (Holmes and Rahe, 1967). Whether exposed individuals go on to develop symptoms of psychiatric symptoms, however, is highly dependent on the availability of social support or lack thereof (Kilpatrick et al., 2007; McQuaid et al., 2016). Indeed, social support and physical contact have shown measurable consequences on physiological parameters in reducing stress-induced cortisol and norepinephrine levels, as well as blood pressure (Grewen et al., 2005). These beneficial effects are possibly moderated by the oxytocin system, as OXTR rs53576 G allele homozygotes seem to particularly benefit from social support (Chen et al., 2011b). Several studies also support that polymorphisms in oxytocin system genes moderate social sensitivity (McInnis et al., 2015, 2017). As previously noted, the very availability of social support to an individual might be mediated by OXTR genotype. Similar to their younger counterparts, adult individuals homozygous for the OXTR rs53576 G allele exhibited higher empathy (Rodrigues et al., 2009), were judged to be more prosocial by others (Kogan et al., 2011), and ultimately had a higher tolerance for stress than individuals carrying an A allele (Lucas-Thompson and Holman, 2013). Differential sensitivity to adult stressful life events has also been reported for the other candidate genes, as reviewed in Halldorsdottir and Binder (2017). For adult life events, not only candidate genes have been investigated, but also genomewide interaction patterns using different strategies. For example, polygenic risk scores derived from GWAS for MDD show inconsistent interactions with stressful life events in predicting depressive symptoms (Mullins et al., 2016; Peyrot et al., 2017). On the other hand, polygenic risk scores for bipolar disorder and schizophrenia interacted with trauma exposure to predict alcohol abuse in more than 10,000 US-Army soldiers (Polimanti et al., 2018b). In addition to mapping polygenic risk scores, gene-environment wide interaction studies (GEWIS) have been attempted. In study within more than 10,000 participants, GEWIS identified a SNP 14 kb from CEP350 interacting with stressful life events to predict depressive symptoms in the larger African-American subset (Dunn et al., 2016). The functionality of CEP350 is not well characterized yet, but it is known to be involved in the organization of microtubules and may therefore play a role in adult neuroplasticity. Another GEWIS in two independent cohorts of over 24,000 individuals identified SNPs in PRKG1, a gene involved in learning and memory, to interact with trauma exposure to predict alcohol abuse (Polimanti et al., 2018a). Genes moderating the capacity of neural plasticity may thus be relevant candidates for resilience mechanisms in adulthood.

Conclusions We here outlined how genetic factors in interaction with changing environmental stressors shape the developing brain toward disease or resilience. Studying gene  environment interactions may thus be informative for understanding resilience mechanisms providing information on relevant molecular and cellular mechanisms, brain circuits, and behavioral strategies. A detailed mapping of gene  environment interactions in large longitudinal cohorts with repeated biological, neuroimaging, behavioral, and symptomatic measures may allow to dissect mechanisms of resilience at different developmental stages and to inform strategies for enhancing resilience.

204

14. How genes and environment interact to shape risk and resilience to stress-related psychiatric disorders

References Alboni, S., Van Dijk, R.M., Poggini, S., Milior, G., Perrotta, M., Drenth, T., et al., 2017. Fluoxetine effects on molecular, cellular and behavioral endophenotypes of depression are driven by the living environment. Molecular Psychiatry 22 (4), 552. Angold, A., Costello, E.J., Erkanli, A., &Worthman, C.M., 1999. Pubertal changes in hormone levels and depression in girls. Psychological Medicine 29 (5), 1043e1053. American Psychological Association, May 17, 2018. The Road to Resilience. Retrieved from. http://www.apa.org/ helpcenter/road-resilience.aspx. Apter-Levy, Y., Feldman, M., Vakart, A., Ebstein, R.P., Feldman, R., 2013. Impact of maternal depression across the first 6 years of life on the child’s mental health, social engagement, and empathy: the moderating role of oxytocin. American Journal of Psychiatry 170 (10), 1161e1168. Baker, M., Lindell, S.G., Driscoll, C.A., Zhou, Z., Yuan, Q., Schwandt, M.L., et al., 2017 Oct 31. Early rearing history influences oxytocin receptor epigenetic regulation in rhesus macaques. Proceedings of the National Academy of Sciences USA 114 (44), 11769e11774. Bandura, A., Ross, D., Ross, S.A., 1963. Imitation of film-mediated aggressive models. Journal of Abnormal and Social Psychology 66 (1), 3e11. Baribeau, D.A., Dupuis, A., Paton, T.A., Scherer, S.W., Schachar, R.J., Arnold, P.D., et al., 2017. Oxytocin receptor polymorphisms are differentially associated with social abilities across neurodevelopmental disorders. Scientific Reports 7 (1), 11618. Barrett, C.E., Arambula, S.E., Young, L.J., 2015. The oxytocin system promotes resilience to the effects of neonatal isolation on adult social attachment in female prairie voles. Translational Psychiatry 5 (7), e606. Blair, C., Raver, C.C., 2012. Child development in the context of adversity: experiential canalization of brain and behavior. American Psychologist 67 (4), 309. Bowlby, J., 1954. The effect of separation from the mother in early life. Irish Journal of Medical Science 29 (3), 121e126 (1926-1967). Brannigan, R., Cannon, M., Tanskanen, A., Huttunen, M.O., Leacy, F.P., Clarke, M.C., 2018. The association between subjective maternal stress during pregnancy and offspring clinically diagnosed psychiatric disorders. Acta Psychiatrica Scandinavica. Brody, G.H., Beach, S.R., Philibert, R.A., Chen, Y.F., Murry, V.M., 2009. Prevention effects moderate the association of 5-HTTLPR and youth risk behavior initiation: gene environment hypotheses tested via a randomized prevention design. Child Development 80 (3), 645e661. Cattaneo, A., Cattane, N., Malpighi, C., Czamara, D., Suarez, A., Mariani, N., et al., 2018, Nov. FoxO1, A2M, and TGF-b1: three novel genes predicting depression in gene X environment interactions are identified using crossspecies and cross-tissues transcriptomic and miRNomic analyses. Molecular Psychiatry 1, 23 (11), 2192e2208. Chen, F.S., Kumsta, R., von Dawans, B., Monakhov, M., Ebstein, R.P., Heinrichs, M., 2011b. Common oxytocin receptor gene (OXTR) polymorphism and social support interact to reduce stress in humans. Proceedings of the National Academy of Sciences 108 (50), 19937e19942. Chen, F.S., Barth, M., Johnson, S.L., Gotlib, I.H., Johnson, S.C., 2011a. Oxytocin receptor (OXTR) polymorphisms and attachment in human infants. Frontiers in Psychology 2, 200. Cicchetti, D., Rogosch, F.A., 2012. Gene Environment interaction and resilience: effects of child maltreatment and serotonin, corticotropin releasing hormone, dopamine, and oxytocin genes. Development and Psychopathology 24 (2), 411e427. Cline, J.I., Belsky, J., Li, Z., Melhuish, E., Lysenko, L., McFarquhar, T., et al., 2015. Take your mind off it: coping style, serotonin transporter linked polymorphic region genotype (5-HTTLPR), and children’s internalizing and externalizing problems. Development andpsychopathology 27 (4pt.1), 1129e1143. Dannlowski, U., Stuhrmann, A., Beutelmann, V., Zwanzger, P., Lenzen, T., Grotegerd, D., et al., 2012. Limbic scars: long-term consequences of childhood maltreatment revealed by functional and structural magnetic resonance imaging. Biological Psychiatry 71 (4), 286e293. Deng, W., Aimone, J.B., Gage, F.H., 2010. New neurons and new memories: how does adult hippocampal neurogenesis affect learning and memory? Nature Reviews Neuroscience 11 (5), 339. Di Iorio, C.R., Carey, C.E., Michalski, L.J., Corral-Frias, N.S., Conley, E.D., Hariri, A.R., Bogdan, R., 2017. Hypothalamic-pituitary-adrenal axis genetic variation and early stress moderates amygdala function. Psychoneuroendocrinology 80, 170e178.

References

205

Dunn, E.C., Wiste, A., Radmanesh, F., Almli, L.M., Gogarten, S.M., Sofer, T., et al., 2016. Genome-wide association study (GWAS) and genome-wide by environment interaction study (GWEIS) of depressive symptoms in African American and Hispanic/Latina women. Depression and Anxiety 33 (4), 265e280. Eriksson, P.S., Perfilieva, E., Björk-Eriksson, T., Alborn, A.M., Nordborg, C., Peterson, D.A., Gage, F.H., 1998. Neurogenesis in the adult human hippocampus. Nature Medicine 4 (11), 1313. Feng, Y., Su, M., Si, Y.J., Guo, Q.W., Lin, J., Cao, T., et al., 2018. Longitudinal interplays of estrogen receptor alpha gene rs9340799 with social-environmental factors on post-traumatic stress disorder in Chinese Han adolescents after Wenchuan earthquake. American Journal of Medical Genetics Part B: Neuropsychiatric Genetics 177 (3), 337e345. Frodl, T., Reinhold, E., Koutsouleris, N., Donohoe, G., Bondy, B., Reiser, M., et al., 2010. Childhood stress, serotonin transporter gene and brain structures in major depression. Neuropsychopharmacology 35 (6), 1383. Gogtay, N., Giedd, J.N., Lusk, L., Hayashi, K.M., Greenstein, D., Vaituzis, A.C., et al., 2004. Dynamic mapping of human cortical development during childhood through early adulthood. Proceedings of the National Academy of Sciences of the United States of America 101 (21), 8174e8179. Grabe, H.J., Wittfeld, K., Van der Auwera, S., Janowitz, D., Hegenscheid, K., Habes, M., et al., 2016. Effect of the interaction between childhood abuse and rs1360780 of the FKBP5 gene on gray matter volume in a general population sample. Human Brain Mapping 37 (4), 1602e1613. Grewen, K.M., Girdler, S.S., Amico, J., Light, K.C., 2005. Effects of partner support on resting oxytocin, cortisol, norepinephrine, and blood pressure before and after warm partner contact. Psychosomatic Medicine 67 (4), 531e538. Halldorsdottir, T., Binder, E.B., 2017. Gene  environment interactions: from molecular mechanisms to behavior. Annual Review of Psychology 68, 215e241. Halldorsdottir, T., de Matos, A.P.S., Awaloff, Y., Arnarson, E.Ö., Craighead, W.E., Binder, E.B., 2017. FKBP5 moderation of the relationship between childhood trauma and maladaptive emotion regulation strategies in adolescents. Psychoneuroendocrinology 84, 61e65. Hankin, B.L., Nederhof, E., Oppenheimer, C.W., Jenness, J., Young, J.F., Abela, J.R.Z., et al., 2011. Differential susceptibility in youth: evidence that 5-HTTLPR x positive parenting is associated with positive affect ‘for better and worse’. Translational Psychiatry 1 (10), e44. Holmes, T.H., Rahe, R.H., 1967. The social readjustment rating scale. Journal of Psychosomatic Research 11 (2), 213e218. Hunziker, U.A., Barr, R.G., 1986. Increased carrying reduces infant crying: a randomized controlled trial. Pediatrics 77 (5), 641e648. Huo, L., Straub, R.E., Roca, C., Schmidt, P.J., Shi, K., Vakkalanka, R., et al., 2007. Risk for premenstrual dysphoric disorder is associated with genetic variation in ESR1, the estrogen receptor alpha gene. Biological Psychiatry 62 (8), 925e933. Hygen, B.W., Belsky, J., Stenseng, F., Lydersen, S., Guzey, I.C., &Wichstrøm, L., 2015. Child exposure to serious life events, COMT, and aggression: testing differential susceptibility theory. Developmental Psychology 51 (8), 1098. Kang, H.J., Kawasawa, Y.I., Cheng, F., Zhu, Y., Xu, X., Li, M., et al., 2011. Spatio-temporal transcriptome of the human brain. Nature 478 (7370), 483. Kessler, R.C., Berglund, P., Demler, O., Jin, R., Merikangas, K.R., Walters, E.E., 2005. Lifetime prevalence and age-ofonset distributions of DSM-IV disorders in the national comorbidity Survey replication. Archives of General Psychiatry 62 (6), 593e602. Kessler, R.C., McLaughlin, K.A., Green, J.G., Gruber, M.J., Sampson, N.A., Zaslavsky, A.M., et al., 2010. Childhood adversities and adult psychopathology in the WHO World mental health surveys. The British Journal of Psychiatry 197 (5), 378e385. Kilpatrick, D.G., Koenen, K.C., Ruggiero, K.J., Acierno, R., Galea, S., Resnick, H.S., et al., 2007. The serotonin transporter genotype and social support and moderation of posttraumatic stress disorder and depression in hurricaneexposed adults. American Journal of Psychiatry 164 (11), 1693e1699. Kogan, A., Saslow, L.R., Impett, E.A., Oveis, C., Keltner, D., Saturn, S.R., 2011. Thin-slicing study of the oxytocin receptor (OXTR) gene and the evaluation and expression of the prosocial disposition. Proceedings of the National Academy of Sciences 108 (48), 19189e19192. Lucas-Thompson, R.G., Holman, E.A., 2013. Environmental stress, oxytocin receptor gene (OXTR) polymorphism, and mental health following collective stress. Hormones and Behavior 63 (4), 615e624.

206

14. How genes and environment interact to shape risk and resilience to stress-related psychiatric disorders

Mairesse, J., Lesage, J., Breton, C., Bréant, B., Hahn, T., Darnaudéry, M., et al., 2007. Maternal stress alters endocrine function of the feto-placental unit in rats. American Journal of Physiology-Endocrinology and Metabolism 292 (6), E1526eE1533. McInnis, O.A., McQuaid, R.J., Matheson, K., Anisman, H., 2015. The moderating role of an oxytocin receptor gene polymorphism in the relation between unsupportive social interactions and coping profiles: implications for depression. Frontiers in Psychology 6, 1133. McInnis, O.A., McQuaid, R.J., Matheson, K., Anisman, H., 2017. Relations between plasma oxytocin, depressive symptoms and coping strategies in response to a stressor: the impact of social support. Anxiety, Stress & Coping 30 (5), 575e584. McQuaid, R.J., McInnis, O.A., Paric, A., Al-Yawer, F., Matheson, K., &Anisman, H., 2016. Relations between plasma oxytocin and cortisol: the stress buffering role of social support. Neurobiology of stress 3, 52e60. Mehta, D., Eapen, V., Kohlhoff, J., Mendoza Diaz, A., Barnett, B., Silove, D., Dadds, M.R., 2016. Genetic regulation of maternal oxytocin response and its influences on maternal behavior. Neural Plasticity 2016. Mileva-Seitz, V., Steiner, M., Atkinson, L., Meaney, M.J., Levitan, R., Kennedy, J.L., et al., 2013. Interaction between oxytocin genotypes and early experience predicts quality of mothering and postpartum mood. PLoS One 8 (4), e61443. Mullins, N., Power, R.A., Fisher, H.L., Hanscombe, K.B., Euesden, J., Iniesta, R., et al., 2016. Polygenic interactions with environmental adversity in the aetiology of major depressive disorder. Psychological Medicine 46 (4), 759e770. Niwa, M., Jaaro-Peled, H., Tankou, S., Seshadri, S., Hikida, T., Matsumoto, Y., et al., 2013. Adolescent stresseinduced epigenetic control of dopaminergic neurons via glucocorticoids. Science 339 (6117), 335e339. O’Donnell, K.J., Meaney, M.J., 2016. Fetal origins of mental health: the developmental origins of health and disease hypothesis. American Journal of Psychiatry 174 (4), 319e328. Peyrot, W.J., Van der Auwera, S., Milaneschi, Y., Dolan, C.V., Madden, P.A., Sullivan, P.F., et al., 2017. Does childhood trauma moderate polygenic risk for depression? A meta-analysis of 5765 subjects from the psychiatric genomics consortium. Biological Psychiatry. Pfefferbaum, A., Mathalon, D.H., Sullivan, E.V., Rawles, J.M., Zipursky, R.B., Lim, K.O., 1994. A quantitative magnetic resonance imaging study of changes in brain morphology from infancy to late adulthood. Archives of Neurology 51 (9), 874e887. Polimanti, R., Kaufman, J., Zhao, H., Kranzler, H.R., Ursano, R.J., Kessler, R.C., et al., 2018b. Trauma exposure interacts with the genetic risk of bipolar disorder in alcohol misuse of US soldiers. Acta Psychiatrica Scandinavica 137 (2), 148e156. Polimanti, R., Kaufman, J., Zhao, H., Kranzler, H.R., Ursano, R.J., Kessler, R.C., et al., 2018a. A genome-wide gene-bytrauma interaction study of alcohol misuse in two independent cohorts identifies PRKG1 as a risk locus. Molecular Psychiatry 23 (1), 154. Qiu, A., Shen, M., Buss, C., Chong, Y.S., Kwek, K., Saw, S.M., et al., 2017. Effects of antenatal maternal depressive symptoms and socio-economic status on neonatal brain development are modulated by genetic risk. Cerebral Cortex 27 (5), 3080e3092. Qiu, A., Tuan, T.A., Ong, M.L., Li, Y., Chen, H., Rifkin-Graboi, A., et al., 2015. COMT haplotypes modulate associations of antenatal maternal anxiety and neonatal cortical morphology. American Journal of Psychiatry 172 (2), 163e172. Rodrigues, S.M., Saslow, L.R., Garcia, N., John, O.P., Keltner, D., 2009. Oxytocin receptor genetic variation relates to empathy and stress reactivity in humans. Proceedings of the National Academy of Sciences 106 (50), 21437e21441. Sannino, S., Padula, M.C., Managò, F., Schaer, M., Schneider, M., Armando, M., et al., 2017. Adolescence is the starting point of sex-dichotomous COMT genetic effects. Translational Psychiatry 7 (5), e1141. Seth, S., Lewis, A.J., Saffery, R., Lappas, M., Galbally, M., 2015. Maternal prenatal mental health and placental 11b-HSD2 gene expression: initial findings from the mercy pregnancy and emotional wellbeing study. International Journal of Molecular Sciences 16 (11), 27482e27496. Sharp, H., Pickles, A., Meaney, M., Marshall, K., Tibu, F., Hill, J., 2012. Frequency of infant stroking reported by mothers moderates the effect of prenatal depression on infant behavioural and physiological outcomes. PLoS One 7 (10), e45446.

References

207

Sharp, H., Hill, J., Hellier, J., Pickles, A., 2015. Maternal antenatal anxiety, postnatal stroking and emotional problems in children: outcomes predicted from pre-and postnatal programming hypotheses. Psychological Medicine 45 (2), 269e283. Steinberg, L., Chein, J.M., 2015. Multiple accounts of adolescent impulsivity. Proceedings of the National Academy of Sciences 112 (29), 8807e8808. Sullivan, P.F., Daly, M.J., O’Donovan, M., 2012. Genetic architectures of psychiatric disorders: the emerging picture and its implications. Nature Reviews Genetics 13 (8), 537. Thompson, R.J., Parker, K.J., Hallmayer, J.F., Waugh, C.E., Gotlib, I.H., 2011. Oxytocin receptor gene polymorphism (rs2254298) interacts with familial risk for psychopathology to predict symptoms of depression and anxiety in adolescent girls. Psychoneuroendocrinology 36 (1), 144e147. Ursini, G., Punzi, G., Chen, Q., Marenco, S., Robinson, J.F., Porcelli, A., et al., 2018. Convergence of placenta biology and genetic risk for schizophrenia. Nature Medicine 24 (6), 792. Van den Bergh, B.R., van den Heuvel, M.I., Lahti, M., Braeken, M., de Rooij, S.R., Entringer, S., et al., 2017. Prenatal developmental origins of behavior and mental health: the influence of maternal stress in pregnancy. Neuroscience & Biobehavioral Reviews. Velders, F.P., Dieleman, G., Henrichs, J., Jaddoe, V.W., Hofman, A., Verhulst, F.C., et al., 2011. Prenatal and postnatal psychological symptoms of parents and family functioning: the impact on child emotional and behavioural problems. European Child & Adolescent Psychiatry 20 (7), 341e350. Wang, C., Shen, M., Guillaume, B., Chong, Y.S., Chen, H., Fortier, MV., Meaney, M.J., Qiu, A., et al., 2018 Feb. FKBP5 Moderates the Association between Antenatal Maternal Depressive Symptoms and Neonatal Brain Morphology. Neuropsychopharmacology 43 (3), 564e570. https://doi.org/10.1038/npp.2017.232. Epub 2017 Oct 3. PubMed PMID: 28975925; PubMed Central PMCID: PMC5770768. Zimmermann, P., Spangler, G., 2016. Effects of Gene attachment interaction on adolescents’ emotion regulation and aggressive hostile behavior towards their mothers during a computer game. Frontiers in Human Neuroscience 10, 254.

C H A P T E R

15

Molecular characterization of the resilient brain: transcriptional and epigenetic mechanisms Orna Issler, Zachary S. Lorsch, Eric J. Nestler Nash Family Department of Neuroscience and Friedman Brain Institute, Icahn School of Medicine at Mount Sinai, New York, NY, United States

Introduction There are profound individual differences in the response to stress. In some people, stress induces a range of behavioral abnormalities, whereas in the majority of individuals, exposure to the same stressor has no discernible effect. This variability contributes to differing susceptibility to several psychiatric disorders such as major depressive disorder (MDD), during the development of which only a subset of individuals exposed to chronic stress will exhibit clinical symptoms (reviewed in (Russo et al., 2012; Franklin et al., 2012; Feder et al., 2009)). In this context, the individuals who do not develop behavioral changes that impede social and occupational functioning, despite exposure to severe stress, are termed “resilient.” Because stress in life is inevitable, understating what makes specific individuals stress resilient is of great interest, as this knowledge could provide a foundation for efforts to prevent and treat syndromes such as MDD. Of particular interest is elucidating the molecular processes that underlie resilience, as these molecular changes could be mimicked by therapeutics to treat or prevent pathological states in susceptible individuals. In the laboratory, animal models involving controlled applications of different types of stress can be used to explore individual differences in behavioral and physiological responses (Fig. 15.1). These models are predicated on individual differences in stress reactivity, which result from genetic, epigenetic, or other factors. For example, selective breeding of rodents has resulted in distinct reactive and nonreactive strains that show altered behavioral readouts (Touma et al., 2008; Stead et al., 2006) and molecular profiles (Chaudhury et al., 2014; Hamilton et al., 2014). Other models such as chronic social defeat stress (CSDS) (Berton et al., 2006; Krishnan et al., 2007), chronic variable stress (CVS) (Ota et al., 2014), and learned Stress Resilience https://doi.org/10.1016/B978-0-12-813983-7.00015-X

209

Copyright © 2020 Elsevier Inc. All rights reserved.

210 15. Molecular characterization of the resilient brain: transcriptional and epigenetic mechanisms

FIGURE 15.1 Experimental framework to identify and validate molecular changes associated with stress resilience in animal models. A chronic stress paradigm, stain differences, or selective breeding of rodents are used to induce depression-like behavioral abnormalities in individual animals. Such abnormalities are assessed using a battery of behavioral and neuroendocrine tests, which can differentiate susceptible versus resilient phenotypes in some paradigms. Brain, blood, and other peripheral tissue are collected from control, susceptible, and resilient animals and analyzed to identify molecular and biochemical changes. This can occur either by a candidate gene approach or through unbiased, high-throughput screens. Bioinformatic tools are then utilized to integrate and compare the molecular changes associated with resilience and to identify key target genes and molecular pathways. Finally, it is essential that such predictions are validated in vivo to confirm a causal role in mediating stress resilience. Reprinted with permission from © Mount Sinai Health System.

Introduction

211

helplessness (Su et al., 2016), among others, utilize inbred mouse lines that show individual variations in stress reactivity (Box 15.1). As these mice share a common genetic background, models such as these are useful to pinpoint epigenetic changes associated with resilience. BOX 15.1

Chronic stress paradigms in rodents Depression is a devastating psychiatric disorder that affects nearly 10% of the population. Individuals with depression suffer from a range of emotional, cognitive, and physiological symptoms such as sadness, anhedonia, guilt, energy changes, problems concentrating, changes in appetite, psychomotor alterations, and thoughts of suicide (Akil et al., 2017; Nestler, 2014). Given the heterogeneity of the human depression syndrome, and the prominence of many abnormalities that are uniquely human, it is impossible to fully recapitulate depression in a rodent (Nestler and Hyman, 2010). Rather, the goal is to expose rodents repeatedly to different types of stresses and identify those individuals (susceptible) that develop behavioral abnormalities that are reminiscent of human depression and other individuals (resilient) that avoid some or all of these abnormalities and maintain normal behavioral and physiological functioning despite the stress. Ultimately, it is essential to validate findings from rodent models in humans through the study of postmortem brain tissue. The following are examples of chronic stress paradigms used to study resilience. CSDS is a widely used procedure to distinguish between susceptibility and resilience in mice (Berton et al., 2006; Krishnan et al., 2007). In this procedure, a C57BL/6 mouse is placed into the home cage of a bigger retired CD-1 breeder. During this session, the intruder mouse is subjected to physical aggression by the CD-1 for 5 e10 min. Next, the two mice are separated by a perforated divider for the remainder of the

day. This divider prevents any further physical interactions but exposes the C57BL/6 mouse continuously to the aggressive mouse. This procedure is repeated daily for a total of 10 days, with a new aggressor each day (see (Golden et al., 2011) for protocol). Several assays are used to measure behavioral abnormalities after CSDS. The social interaction (SI) test is used to probe social avoidance. In this test, mice are allowed to explore an arena twice. First, while alone, and second, with a novel CD-1 breeder mouse that is placed in a confined space called the interaction zone. Susceptible mice avoid interacting with the novel mouse, whereas control and resilient mice spend more time in the interaction zone when the novel mouse is present than when the confined space is empty (Krishnan et al., 2007). Importantly, the social avoidance exhibited by susceptible mice generalizes to all mice, including conspecific C57BL/6 mice. Susceptibility versus resilience established by the social interaction test correlates strongly with performance in several other assays, such as sucrose preference, sexual behavior and high-fat food consumption. (Berton et al., 2006; Krishnan et al., 2007). Although CSDS was developed originally for male mice, several paradigms have been established for females (Harris et al., 2017; Steinman and Trainor, 2017; Takahashi et al., 2017). CSDS has proven particularly useful for three main reasons. First is its ability to so clearly differentiate susceptible versus resilient subpopulations of animals. Second, many of the behavioral abnormalities induced by CSDS are essentially permanent, Continued

212

15. Molecular characterization of the resilient brain: transcriptional and epigenetic mechanisms

BOX 15.1 which makes it possible to test the ability of experimental manipulations to reverse stressinduced abnormalities (Berton et al., 2006; Tsankova et al., 2006). This is in contrast to several other paradigms, which induce more transient behavioral abnormalities and can only be used to study the ability of a manipulation to prevent the deleterious effects of concomitant stress. Finally, standard antidepressants exert therapeutic-like effects after CSDS only with chronic administration, and with acute antidepressant-like effects seen with ketamine as observed in humans. (Donahue et al., 2014). CVS refers to several procedures, also known as chronic unpredictable stress or chronic mild stress, in which rodents are exposed to different stresses each day (Willner et al., 1987; Hill et al., 2012). The stress regime is performed for several weeks and comprises stressors ranging from cage tilting and/or flooding in the milder protocols, and foot shock and/or restraint stress in the more harsh protocols. The usage of multiple stressors in random order is to prevent habituation to the stressors. CVS has been shown to induce increased depression-like behaviors in rodents, particularly anhedonia (Willner et al., 1992). However, there are also reports of no effect or decreased depressivelike phenotypes following CVS (Willner, 2005). A strength of the CVS model is that it can be performed in multiple age groups. There are also sex differences in the effects of CVS, as a shorter stress exposure is sufficient to induce a depressive phenotype in female but not male mice (Hodes et al., 2015). These characteristics make CVS a compelling model

(cont'd) for studying the increased risk of females for mood disorders and the relative resilience of males to this particular type of stress. Learned helplessness is a model in which exposure of rodents to repeated inescapable shock leads to the development of passive responses to future shocks in a subset of animals (Maier, 1984). Such passive responses are associated with cognitive, motivational, and emotional deficits and have also been linked to certain neurobiological changes that mimic aspects of depression (Willner, 1986). Individual differences in escape latency have been used as proxy for stress susceptibility and resilience (Berton et al., 2007; Su et al., 2016). Early-life stress: Exposure to stress in early life, such as neglect or abuse, increases the risk for developing depression later in life (Felitti et al., 1998). This phenomenon is mimicked in rodents by exposure to stressors such maternal separation or reduced cage nesting materials in critical periods in early life (Francis et al., 1999; Heim et al., 2008; Bale et al., 2010). In some protocols, subsequent exposure to stress in adulthood is needed to reveal the enhanced stress susceptibility (Pena et al., 2017). Notably, there are reports that maternal care contributes to stress resilience by long-term programming of the offspring’s stress responses (Liu et al., 1997; Korosi et al., 2010). Several other chronic stress or other procedures (e.g., chronic corticosterone administration) have been used to study stress-related pathologies, but have not been used widely to date to study resilience.

Introduction

213

Using these animal models, a wide range of molecular changes have been associated with stress resilience (Fig. 15.1). In particular, studies have identified resilient-specific changes at the levels of RNA (Krishnan et al., 2007; Bagot et al., 2016), protein (Henningsen et al., 2012; Palmfeldt et al., 2016), chromatin (Wilkinson et al., 2009; Dias et al., 2014), and DNA (Elliott et al., 2010; Feng et al., 2017), all of which can have an impact on brain function. Many studies of resilience to date have examined effects on candidate genes or molecular pathways known to be perturbed in stress-susceptible animals or in human MDD. An increasing number of studies, however, are the result of unbiased genome-wide profiling approaches. Such approaches, when combined with advanced systems biology and bioinformatics analysis, have the potential to reveal novel regulators of stress resilience. For these cases, in vivo validation is essential to provide causal evidence that a given target molecule is indeed pro-resilient. Such validations involve direct genetic manipulations in a given brain region, such as viral-mediated gene transfer, which induce behavioral resilience upon mimicking a molecular change associated with resilience, as well as blocking of behavioral resilience by occluding that molecular change (Hamilton et al., 2017). Such tactics have the potential to establish the involvement of specific pathways in pro-resilient behavior and to provide more mechanistic detail as to the role of the individual molecules and pathways in stress resilience (Fig. 15.1). The molecular changes associated with stress resilience arise from a combination of genetic and environmental factors. To date, most studies of genetic contributions to stress susceptibility have focused on the underlying genetics of MDD with minimal investigation of factors that directly promote resilience. Even though twin studies have long suggested that heritability in MDD approaches 40% (Sullivan et al., 2000), only recently have genomewide studiesdinvolving the examination of tens and hundreds of thousands of subjects, been able to identify variants associated with MDD that achieve genome-wide significance (Hyde et al., 2016) (CONVERGE Consortium, 2015). It is possible that these or other genetic variants apply to resilience, but have not yet been elucidated because it is difficult to clearly define a large population of resilient individuals. Epigenetics refers to a host of biological mechanisms that can explain the complex interactions between life experiences and molecular changes at the cellular level and has been proposed to mediate individual differences in response to stress (Akil et al., 2017). Epigenetic changes through DNA methylation, chromatin modifications, and noncoding RNAs can contribute to alternations in gene expression and therefore affect cellular processes, neural circuits, and behavior (Fig. 15.2). In this chapter, we focus on literature from animal stress models to elaborate upon the known molecular mechanisms that promote stress resilience. We emphasize studies that concentrate on stress exposure in adulthood and the resulting adaptive changes that prevent the development of behavioral abnormalities reminiscent of MDD. We highlight only the literature that specifically studies the mechanisms of stress resilience and not that of stress susceptibility. Furthermore, we differentiate between resilience, which we associate with active molecular processes that help to mitigate the deleterious influences of stress, from antidepressant-induced molecular changes, which reverse deleterious changes associated with stress susceptibility. Finally, we organize our review by molecular mechanism, examining the broad categories of changes that have been suggested to be involved in stress resilience.

214

15. Molecular characterization of the resilient brain: transcriptional and epigenetic mechanisms

FIGURE 15.2 Epigenetic processes that influence gene expression. Simplified schematic representing epigenetic modifications. DNA exists across the spectrum from tightly spaced nucleosomes (repressed heterochromatin) to more sparsely spaced nucleosomes (active euchromatin), which affects the accessibility of DNA for transcription factor binding and thereby helps set the level of transcription at a given locus. This is regulated in part by DNA modifications such as generally repressive cytosine methylation catalyzed by DNMTs, and generally activating cytosine hydroxymethylation mediated by TET enzymes. Histones tails are also modified in numerous ways, with functional effects on transcription occurring according to the “histone code.” Changes in histone tail methylation are controlled by PRMTs and HMTs, and many HDMs, although changes in histone acetylation are controlled by HATs and HDACs. Noncoding RNAs such as microRNAs (miRNAs) and long-noncoding RNAs (lncRNAs) are additional epigenetic mechanisms that regulate gene expression. MiRNAs are posttranscriptional regulators that act in the cell cytoplasm to repress target genes through mRNA destabilization or translational repression. lncRNAs work throughout the cell and interact with DNA, RNA, or protein and function as scaffolds, decoys, or regulators. DNMT, DNA methyltransferase; HAT, histone acetyltransferases; HDAC, histone deacetylase; HDM, histone demethylases; HMT, histone methyltransferase; lncRNA, long-noncoding RNA; miRNA, microRNA; PRMT, protein arginine methyltransferases; Tet, tet methylcytosine dioxygenase. Reprinted with permission from © Mount Sinai Health System.

These include DNA and chromatin modifications, alterations in transcription factors, immunerelated processes, and molecular changes associated with altered neuronal signaling.

DNA methylation DNA methylation is a key epigenetic process that regulates gene expression through direct modifications to DNA (Fig. 15.2). DNA methylation is generally associated with diminished gene expression whereby unmethylated genes are more available for transcription (Meaney and Szyf, 2005). On the other hand, certain variant forms of methylation, for example, hydroxymethylation, of DNA are thought to activate gene expression (see below).

DNA methylation

215

Several studies suggest a role for changes in DNA methylation in stress resilience. Following CSDS, Elliott et al. found increased methylation in the promoter region of the corticotropin-releasing factor (Crf) gene, involved in stress responses, in control and resilient, but not susceptible, mice in the paraventricular nucleus (PVN) of hypothalamus (Elliott et al., 2010). This increased methylation was associated with lower expression level of Crf, and mimicking this process with shRNA to downregulate Crf in the PVN was pro-resilient in vivo (Fig. 15.3). As such, DNA methylation has the potential to reduce expression of key stress-promoting genes to contribute to stress resilience. Alterations in DNA methylation enzymes have also been implicated in promoting stress resilience. For example, one correlative study using CSDS reported higher levels of DNA

FIGURE 15.3 Examples of site-specific genetic manipulations of target molecules implicated in behavioral resilience. Summary of the role of some of the specific targets that have been implicated in resilience organized by the brain site in which the molecule was evaluated. cKO, conditional knockout; DN, dominant negative; DR, dorsal raphe; Hipp, hippocampus; KD, knockdown; KO, knockout; NAc, nucleus accumbens; OE, overexpression; PVN, paraventricular nucleus; VTA, ventral tegmental area; Ac3I, EGFP-fused CaMKII inhibitory peptide (Robison et al., 2014); Baz1b, bromodomain adjacent to zinc finger domain 1B (Sun et al., 2016); ß-catenin (Dias et al., 2014); Bdnf, brain-derived neurotrophic factor (Taliaz et al., 2011; Duclot and Kabbaj, 2013; Krishnan et al., 2007); Caspase-1, caspase-1/interleukin-1 converting enzyme (Li et al., 2017); Cdk5, cyclin-dependent kinase 5 (Heller et al., 2016); Crf, corticotropin-releasing factor (Elliott et al., 2010); DFosB, delta FosB (Vialou et al., 2010b; Donahue et al., 2014; Vialou et al., 2010a; Berton et al., 2007; Ohnishi et al., 2015); Dnmt3A, DNA methyltransferase 3A (Hodes et al., 2015); Gdnf, glial cellederived neurotrophic factor (Uchida et al., 2011); GluR2, glutamate receptor 2 (Vialou et al., 2010b); Hdac2, histone deacetylase 2 (Uchida et al., 2011); Hdac6, histone deacetylase 6 (Espallergues et al., 2012); Hsp90, heat shock protein 90 (Jochems et al., 2015); IKK, IkB kinase (Christoffel et al., 2011; Christoffel et al., 2012); Otx2, orthodenticle homeobox 2 (Pena et al., 2017); miR-124 (Higuchi et al., 2016); p38a MAPK, P38 mitogen-activated protein kinases (Bruchas et al., 2011); PTPN5, protein tyrosine phosphatase, nonreceptor type 5 (Yang et al., 2012); Smarca5, SWI/ SNF-related, matrix-associated, actin-dependent regulator of chromatin, subfamily A, member 5 (Sun et al., 2016); SC1, Sparc-like 1 (Vialou et al., 2010b); Tet1, tet methylcytosine dioxygenase 1 (Feng et al., 2017); xCT, cystineglutamate antiporter (Nasca et al., 2017). Reprinted with permission from © Mount Sinai Health System.

216

15. Molecular characterization of the resilient brain: transcriptional and epigenetic mechanisms

methyltransferase 3A (Dnmt3a), which methylates DNA, in recently differentiated neurons in the dentate gyrus of resilient mice (Hammels et al., 2015). Using the CVS model, Hodes et al. expanded these findings to show a causal role for Dnmt3a in resilience in female mice. In this experiment, conditional knockout (cKO) of Dnmt3a in the nucleus accumbens (NAc) produced pro-resilient effects after CVS, to which female, but not male mice, are normally more susceptible (Hodes et al., 2015) (Fig. 15.3). Interestingly, cKO of Dnmt3a in the NAc of female mice led to dramatic changes in gene expression that made the transcriptome appear more “male-like” (Hodes et al., 2015). As males are inherently more resilient to CVS, these transcriptional changes likely represent a broad pattern of resilience. Recently, there have been reports on the role of enzymes mediating oxidized forms of DNA methylation, such as hydroxymethylation, as well as demethylation, such as tet methylcytosine dioxygenase 1 (Tet1), in stress resilience (Feng et al., 2017). In this study, Tet1 was found to be downregulated following CSDS in the NAc of susceptible but not resilient mice. However, cKO of Tet1 in NAc had a pro-resilient effect in non-stressed mice (Fig. 15.3). RNAseq data showed an overlap between the transcriptional signature of resilience and cKO of Tet1 in this brain region. Taken together, these studies suggest that DNA methylation processes play a role in supporting stress resilience. However, further work is needed to demonstrate the role of the enzymes mediating this process across brain regions, sexes, and cell types, as well as identifying the key genomic locations where DNA methylation promotes resilience. Whole-genome bisulfate sequencing approaches now make it possible to obtain this insight in an unbiased fashion.

Chromatin modifications Histone acetylation and methylation are pivotal processes in switching between repressed heterochromatin and actively translated euchromatin (Fig. 15.2). Several studies have implicated histone modifications in promoting stress resilience. In one study, researchers profiled global changes in histone acetylation across multiple brain regions following CSDS in rats, and found that resilient rats show patterns of histone acetylation more similar to control than susceptible rats (Kenworthy et al., 2014). Interestingly, however, the authors also observed that levels of enzymes that modify chromatin were altered more in both susceptible and resilient than in control rats, suggesting that social stress induces certain chromatin modifications independent of behavioral outcomes. In accordance with findings of unique histone modifications in resilient mice, global ChIP-chip analysis (an earlier iteration prior to the development of ChIP-seq; ChIP: chromatin immunoprecipitation followed by DNA sequencing) for the histone 3 (H3) dimethyl K9/K27 mark in the NAc of mice exposed to CSDS revealed opposite patterns of activity for stress resilience and susceptibility (Wilkinson et al., 2009). Moreover, susceptible mice that had been treated with the antidepressant imipramine showed a H3K9/H3K27 binding pattern that partly mimicked that of resilient mice. This finding suggests that antidepressants act in part via inducing mechanisms of natural resilience. However, additional studies of H3 subunit dynamics in the NAc have shown a response to chronic stress only in susceptible mice, although they do also demonstrate that blockade of this process promotes resilience in vivo (Lepack et al., 2016).

MicroRNAs

217

Enzymes modifying histones, such as histone deacetylase 6 (Hdac6), have also been implicated in stress resilience. Specifically, Hdac6 has been shown to be downregulated in the dorsal raphe (DR) in serotonergic neurons of resilient mice (Espallergues et al., 2012). Accordingly, cKO of Hadc6 in serotonergic neurons was sufficient to increase resilience and mitigated CSDS-induced electrophysiological and morphological alterations, a process that is believed to occur as a result of Hdac6’s interaction with the glucocorticoid receptor (GR) (Espallergues et al., 2012) (Fig. 15.3). Given the wide-ranging role of chromatin dynamics in resilience, it is perhaps unsurprising that pharmacologic agents that modulate histones, such as the histone acetylation agent N-acetyl-cysteine or histone deacetylase inhibitors administered into any of several brain regions, can increase resilience (Nasca et al., 2017). More recent research has begun to focus on nucleosome positioning and the 3D structure of chromatin in response to chronic stress models in rodents (Jiang et al., 2017; Sun et al., 2015, 2016). For example, the chromatin remodeling factor, BAZ1B, which controls the spacing between nucleosomes, is induced in the NAc selectively in mice that are resilient to CSDS, and upon overexpression in this brain region, promotes behavioral resilience (Sun et al., 2016). ChIP-seq analysis of genomic targets of BAZ1B has provided initial insight into the underlying mechanisms involved. In sum, there is considerable evidence for chromatin modifications playing a role in stress resilience. However, this line of research needs to be expanded by additional ChIP-seq and related genome-wide studies that provide global profiling of chromatin across several limbic brain regions in the context of stress to further elucidate genomic hot spots for resilience.

MicroRNAs Noncoding RNAs play an important regulatory role in cells (Issler and Chen, 2015). In particular, the subclass of microRNAs (miRNAs), which act as posttranscriptional repressors, has been implicated as a molecular mediator of stress resilience across several brain sites. In the NAc, the miRNAome of mice resilient to CSDS shows substantial differences from that of mice susceptible to CSDS (Dias et al., 2014). With regards to specific transcripts, overexpression of miR-135 in serotonergic neurons within the DR increases resilience to CSDS (Issler et al., 2014), whereas overexpression of miR-124 in the hippocampus increases resilience CVS (Higuchi et al., 2016) (Fig. 15.3). Interestingly, both miR-135 and miR-124 are upregulated by antidepressant treatment, but the proposed mechanisms of action for these two transcripts are distinct. Although miR-135 functions in part by repressing serotonin-related genes (Issler et al., 2014), miR-124 has been shown to target histone-modifying enzymes (Higuchi et al., 2016). In addition to miR-124, other miRNA transcripts have also been shown to play a role in stress resilience in the hippocampus, suggesting a broad role for different miRNAs even in the same brain region. For example, miR-455-3p is upregulated and miR30e-3p is downregulated in resilience in rat stress models (Pearson-Leary et al., 2017). Taken together, these diverging effects of stress on different miRNA transcripts suggest that miRNAs have a complex role in resilience. As such, further study of specific miRNA transcripts, as well as other classes of noncoding RNAs, is warranted.

218

15. Molecular characterization of the resilient brain: transcriptional and epigenetic mechanisms

Transcription factors Binding of transcription factors regulates gene expression and partially explain the broad patterns of transcriptional changes seen in stress resilience (Bagot et al., 2016; Bagot et al., 2017). The transcription factor delta FosB (dFosB), a splice product of the FosB immediate early gene, has been shown to be upregulated in several brain regions in resilient animals following chronic stress (Nestler, 2015). For example, in the learned helplessness paradigm, resilient mice with low escape latency show higher levels of dFosB in the periaqueductal gray (PAG) than mice with high escape latency (Berton et al., 2007). Similarly, mice resilient to CSDS have elevated levels of dFosB in the NAc (Vialou et al., 2010b), which is localized more in medium spiny neurons expressing the dopamine receptor type 1 (D1) than in neurons expressing the dopamine receptor type 2 (D2) (Lobo et al., 2013). Viral-mediated gene transfer has provided causal evidence for a role of dFosB in stress resilience (Fig. 15.3). In one study, researchers overexpressed dFosB in the PAG and found it to promote resilient following exposure to inescapable stress, an effect that is mediated by dFosB suppression of substance P expression (Berton et al., 2007). Similarly, overexpression of dFosB in the NAc was found to be pro-resilient after CSDS (Vialou et al., 2010b; Donahue et al., 2014); the proposed mechanism in this case involves induction of the GluA2 AMPA receptor subunit (Vialou et al., 2010b). Interestingly, these effects are cell-type-specific: histone acetylation targeted to the FosB gene via use of synthetic zinc finger proteins in D1 neurons was pro-resilient, whereas targeted repressive histone methylation opposes resilience, with the opposite effects observed upon targeting D2 neurons (Hamilton et al., 2017). The induction of dFosB in the NAc of resilience mice is mediated in part by serum response factor (Srf), an upstream regulator of the FosB gene, which is activated in the NAc of resilient mice, where it was shown to promote behavioral resilience upon its own overexpression (Vialou et al., 2010a). In addition to dFosB, transcription factors affecting well-studied molecular pathways, such as the Wnt signaling pathway, have been implicated in stress resilience. Research has shown that this action is also cell-type-specific, as CSDS increases WNT-ß-catenin activity in D2 neurons of the NAc only, with overexpression of ß-catenin selectively in D2 neurons promoting resilience (Dias et al., 2014). ChIP-seq of ß-catenin showed enrichment for binding sites in the NAc of resilient mice over susceptible mice, and Dicer 1 (which controls the biogenesis of miRNAs) and numerous specific miRNA targets have been proposed as molecular regulators of ß-catenin in this context (Dias et al., 2014). Several transcription factors have to date been implicated in stress resilience across multiple brain regions, and it is likely that resilience is the product of numerous transcription factors that interact with chromatin-modifying mechanisms to fundamentally alter the gene expression profile in many brain regions that influence stress responses. Studies using advanced methods such as ChIP-seq, whole-genome bisulfite sequencing, ATAC-seq, and HiC, in combination with RNA-seq, are needed to identify more transcription factors with a role in stress resilience, as well as to define specific molecular interactions that result in the unique global transcriptional profile of resilience (Fig. 15.4).

Transcription factors

219

FIGURE 15.4 Schematic model of theoretical molecular changes associated with stress resilience. Behavioral resilience is defined by the preservation of a normal control-like phenotype despite stress exposure. For individual molecules, this process can be defined by the restoration of control levels of susceptibility-associated changes (patterns (A) and (B)); by a gradient whereby pro-resilient adaptations are in the opposite direction of susceptibilityassociated changes (patterns (C) and (D)); or by changes that are unique to the resilient state (patterns (E )and (F)). The sum of these many changes, all of which have been observed experimentally, is that the molecular profile of stress resilience is not merely the restoration of the control state, but instead a unique, active adaptive response.

220

15. Molecular characterization of the resilient brain: transcriptional and epigenetic mechanisms

Immune-related processes Alternations of the peripheral and central immune system have been linked to stress resilience across multiple studies. Within the brain, microglia, which are derived from peripheral macrophages, play a role in immune surveillance and synaptic pruning and have been implicated in resilience. For example, one study showed that although susceptible mice have elevated levels of microglia and proinflammatory cytokines in the prefrontal cortex (PFC), resilient mice have levels similar to those of nonstressed controls (Couch et al., 2013). Additionally, several independent studies have shown that KO of the chemokine receptor Cx3cr1, which is expressed in the brain exclusively in microglia, promotes stress resilience (Hellwig et al., 2016; Rimmerman et al., 2017; Milior et al., 2016) by altering the expression of immunity-related genes, as well as synaptic function. Peripheral immune regulators also have been implicated in resilience. Lower levels of the circulating proinflammatory cytokine IL-6 were found to be predictive of individual stress resilience in mice (Hodes et al., 2014). Interestingly, inhibiting IL-6 in the periphery by IL-6 KO, bone marrow transplantation from an IL-6 KO mouse, or treating mice systemically with antibodies against IL6, increases behavioral resilience to CSDS (Hodes et al., 2014). Similarly, transferring lymphocytes from mice that are resilient to stress to stress-naïve mice confers resilience, and is associated with increased hippocampal neurogenesis, reduced levels of proinflammatory cytokines and blunted microglia reactivity (Brachman et al., 2015). Together, these findings provide evidence of the interplay between the immune system and the brain in the context of stress resilience. As such, the study of potential mechanisms and treatment avenues for stress-related disorders extends beyond the brain and should not be limited to targeting CNS mechanisms.

Neurotrophic factors Neurotrophic factors such as brain-derived neurotrophic factor (Bdnf) and glial cellederived neurotrophic factor (Gdnf) have been shown to be important for stress resilience across several brain regions. For example, in the VTA-NAc projection, lower levels of Bdnf have been associated with stress resilience (Krishnan et al., 2007). Within the NAc, expression levels of Bdnf in resilient mice are similar to control levels, but susceptible mice show Bdnf induction in response to CSDS. Knocking down Bdnf in the NAc does not affect stress resilience, but knocking down Bdnf in the VTA is sufficient to increase behavioral resilience (Berton et al., 2006). Moreover, mutant mice homozygous for the human variant Met/Met polymorphism (compared with the wild-type Val/Val at amino acid 66)dwhich display decreased Bdnf function in the VTA-NAc pathwaydalso show enhanced stress resilience (Krishnan et al., 2007). In contrast to the mesolimbic dopamine system, Bdnf has been shown to be pro-resilient in the hippocampus across multiple studies (Duman et al., 1997; Monteggia et al., 2004, 2007; Taliaz et al., 2011; Duman, 2014). In rats, overexpression of Bdnf in the hippocampus increases resilience to CVS, and Bdnf knockdown in this region promotes susceptibility (Taliaz et al., 2011). This may be the result of epigenetic changes in Bdnf exon VI, which results in baseline upregulation of Bdnf in resilient animals (Duclot and Kabbaj, 2013).

Circuit-related molecules

221

The opposing effects of Bdnf in different brain sites may be due to the opposite consequences of its effects on strengthening glutamatergic synapses in different brain regions, as well as its unique role in promoting neurogenesis in the hippocampus. Gdnf levels are reported to vary in mouse strains with different inherent levels of stress susceptibility. C57BL/6 mice, which are more resilient to CVS, show higher levels of Gdnf in the striatum than BALB/c mice, which are more susceptible to this form of stress (Uchida et al., 2011). Accordingly, overexpression of Gdnf in the NAc of BALB/c mice is pro-resilient. It has been proposed that these differences in Gdnf may be a result of epigenetic changes at the Gdnf promoter in resilient mice (Uchida et al., 2011).

Circuit-related molecules Stress resilience and susceptibility are associated with numerous circuit-level changes, and specific molecular regulators have been directly implicated in mediating changes in neuronal circuitry in the context of stress. For example, susceptibility to CSDS has been linked to increased phasic firing in VTA dopamine neurons that project to the NAc, a phenomenon not seen in resilient mice (Chaudhury et al., 2013). Optogenetic suppression of the activity of this circuit promotes resilience, whereas its activation mimics susceptibility. The increased excitability of VTA dopamine neurons seen in susceptible mice is mediated in part by increased hyperpolarization-activated cation current (Ih), and direct inhibition of Ih in these neurons exerts pro-resilient and antidepressant-like effects (Cao et al., 2010). Paradoxically, resilient mice show an even larger increase in Ih current, as well as activation of several potassium channels, as compared with either control or susceptible mice, demonstrating that resilient mice have unique adaptations in several types of ion channels that actively promote behavioral resilience (Friedman et al., 2014). Importantly, very different adaptations occur in VTA dopamine neurons in response to different types of chronic stress (see (Tye et al., 2013)), underscoring the importance of considering the type, duration, and severity of the stress used in a rodent model. Molecular regulation of different inputs to the VTA-NAc reward circuit has been shown to contribute to resilience. Optogenetic stimulation of ventral hippocampus (vHIP) inputs to the NAc increases susceptibility to CSDS, while stimulating inputs to the NAc from either the PFC or basolateral amygdala (BLA) promotes resilience (Bagot et al., 2015). Accordingly, molecules that enhance signaling, such as sidekick cell adhesion molecule 1 (Sdk1), which increases the frequency of spontaneous excitatory postsynaptic currents (EPSCs) in affected pyramidal neurons, increases susceptibility when overexpressed in the vHIP, but resilience when overexpressed in the PFC (Bagot et al., 2016). Molecular adaptations seen in the context of behavioral resilience have been associated with changes in neuronal morphology, particularly dendritic spines, in several brain regions. For example, Yang et al. (2012) identified reduced dendritic spine density in the rat hippocampus 1 week after acute stress in susceptible, but not resilient, rats (Yang et al., 2012). By examining levels of protein tyrosine phosphatase nonreceptor 5 (PTPN5) in this region, they found that spine density was positively correlated with PTPN5 expression, which was significantly reduced in susceptible compared with resilient rats. Furthermore, shRNA knockdown of PTPN5 produced a susceptible-like reduction in spine density, whereas overexpression of PTPN5 mimicked the spine density profile of resilience. However, the

222

15. Molecular characterization of the resilient brain: transcriptional and epigenetic mechanisms

consequence of changes in spine density in affecting susceptibility-resilience is region specific. In the NAc, CSDS increases spine density in susceptible compared with resilient mice (Christoffel et al., 2011). This effect was driven largely by increases in immature stubby spines and was associated with higher frequencies of mini EPSCs in susceptible versus resilient mice. These morphological and functional changes were linked to increased expression of IkB kinase (IKK), a downstream target of Bdnf. Viral overexpression of a dominant negative IKK mutant in NAc was sufficient to reverse CSDS-induced increases in spine density in previously susceptible mice and to enhance behavioral resilience. Similarly, in the BLA, rats susceptible to a prolonged course of restraint stress showed increased dendritic arborizations (Vyas et al., 2006). Other studies have shown that repeated stress is associated with BLA hyperexcitability and that this process is not seen in resilient rats (Hetzel and Rosenkranz, 2014). These changes in morphology and electrophysiology may be a direct result of neurotransmitter function, as mutations of the NMDA receptor N2A subunit, but not the AMPA GluA1 subunit, reduce dendritic spine density in the BLA and are associated with a reduction in anxiety-like responses to stress in mice (Mozhui et al., 2010). Taken as a whole, these studies indicate that dendritic density in the BLA is inversely related to stress resilience, with progress being made in identifying some of the molecular mediators involved. Glutamatergic modifications in other brain regions have been functionally linked to stress resilience. In the hippocampus, higher expression of the AMPA receptor subunit GluA1 and decreased AMPA binding have been associated with behavioral measures of stress resilience in mice, with polymorphisms in the GluA1 gene correlating with stress susceptibility (Schmidt et al., 2010). Similarly, GluA2 knockout increases susceptibility and individual variations in GluA2 levels in the hippocampus explain endophenotype differences among high-susceptible and low-susceptible mice following CVS (Nasca et al., 2015). Interestingly, inhibitory circuitry appears to play a role in stress resilience; for example, heterozygous mutations in the GAD65 GABA-synthesizing enzyme confer resilience to behavioral stress in mice (Muller et al., 2014). Finally, as noted earlier, DFosB-mediated induction of GluA2 in D1 medium spiny neurons of NAc promotes behavioral resilience (Vialou et al., 2010b). These findings provide a mechanism by which individual molecular changes can directly alter neuronal morphology and circuit-level function to affect pro-resilient behavior.

Genome-wide studies As mentioned throughout this chapter, high-throughput profiling approaches are now being utilized in the study of resilience. Unlike candidate gene approaches, tools such as microarrays, RNA-seq, ChIP-seq, and several other sequencing-based methods, and mass spectroscopy provide an unbiased view on the molecular, epigenetic, and biochemical landscape associated with stress resilience within individual brain regions of interest. Such studies are a great resource for future investigations, particularly because they identify large-scale patterns of changes that can pinpoint similarities and differences across multiple brain sites and behavioral phenotypes. To date, however, these studies are limited in number (Table 15.1). A major need moving forward is to extend these genome-wide studies to individual cell types within a targeted brain region, to capture the cell typeespecific changes that are likely associated with resilience in different types of neurons, glia, and endothelial cells.

223

Future directions

TABLE 15.1

Studies using high-throughput methods for profiling molecular changes associated with chronic stress resilience in animal models.

Stress model

Animal Brain site model tested Platform

CSDS

Male mice

NAc VTA

CSDS

Male mice

NAc PFC RNA-seq AMY Hip

A larger number of genes are regulated in resilience than Bagot et al. (2016) in susceptibility. There is a high degree of correspondence between gene changes in the NAc and PFC in resilient mice.

CSDS

Male mice

NAc PFC RNA-seq AMY Hip

The transcriptional signature of the antidepressants imipramine and ketamine is similar to that of resilient mice, particularly in the PFC.

Bagot et al. (2017)

CSDS

Male mice

NAc

RNA-seq

The blockade of H3.3 in the NAc promotes resilience and is mediated via regulation of synaptic-related genes.

Lepack et al. (2016)

CSDS

Male mice

NAc

RNA-seq

There is a strong similarity between genes regulated in resilience and genes affected by cKO of Tet1.

Feng et al. (2017)

CSDS

Male mice

NAc

Small RNA-seq

B-catenin-dependent microRNA regulation is associated with resilience.

Dias et al. (2014)

CSDS

Male mice

NAc

ChIP-seq

There is more binding of B-catenin to gene promoters in resilient mice versus susceptible mice.

Dias et al. (2014)

CSDS

Male mice

NAc

ChIP-seq

Increased DNA binding by BAZ1B in resilient mice compared to both control and susceptible.

Sun et al. (2016)

CSDS

Male mice

NAc

ChIP-seq

The binding profile of CREB and the H3 methylation profile in resilient mice is different from that of susceptible mice, but overlaps with that of imipramine treatment.

Wilkinson et al. (2009)

SCVS

Female NAc mice

RNA-seq

cKO of Dnmt3a in females is pro-resilient and shifts the transcriptome to more male-like state.

Hodes et al. (2015)

CUS

Male mice

Hip

RNA-seq

Cx3cr1 KO mice are stress resilient and this is associated with changes in interferon, MHC class I and estrogen receptor signaling pathways.

Rimmerman et al. (2017)

AUS

Male rats

PFC Hip

microarray There is low overlap in genes regulated between resilient, Benatti et al. susceptible, and control groups. (2012)

Repeated Male restraint mice

Main findings

microarray A larger number of genes are regulated in resilience than in susceptibility. The transcriptional profile of susceptible and resilient mice shows limited overlap.

Hip AMY RNA-seq PFC

Strain differences associated with stress resilience and with robust changes in baseline gene expression. Stress is associated with induction of genes associated with the neurotransmitter glutamate.

Reference Krishnan et al. (2007)

Mozhui et al. (2010)

(Continued)

224

15. Molecular characterization of the resilient brain: transcriptional and epigenetic mechanisms

TABLE 15.1

Studies using high-throughput methods for profiling molecular changes associated with chronic stress resilience in animal models.dcont'd

Stress model

Animal Brain site model tested Platform

CMS

Male rats

Hip

microarray Resilient rats show increased expression of immunerelated and signaling-related genes.

Bergstrom et al. (2007)

CMS

Male rats

Hip

MS

The global protein expression profile is different in resilient and susceptible rats. Resilient rats show increased activity related to oxidative phosphorylation.

Henningsen et al. (2012)

CMS

Male rats

PFC

MS

Protein changes associated with resilience involve glutamatergic transmission, the Na/K-transporter and cellular respiration.

Palmfeldt et al. (2016)

Main findings

Reference

AMY, amygdale; AUS, acute unavoidable stress; ChIP-seq, chromatin immunoprecipitation followed by DNA sequencing; cKO, conditional knockout; CMS, chronic mild stress; CSDS, chronic social defeat stress; CUS, chronic unpredictable stress; Cx3cr1, CX3C chemokine receptor 1; Dnmt3A, DNA methyltransferase 3A; H3, histone 3; Hip, hippocampus; KO, knockout; MS, mass spectrometry; NAc, nucleus accumbens; PFC, prefrontal cortex; RNA-seq, RNA sequencing; SCVS, subchronic variable stress; Tet1, tet methylcytosine dioxygenase 1; VTA, ventral tegmental area.

Future directions The last decade has been very fruitful in terms of elaborating the molecular changes that define stress resilience. However, additional mechanistic studies are needed. For example: - There are profound sex differences in stress-induced psychiatric disorders, with females being more likely than males to develop these diseases (Hodes et al., 2017). However, most of the mechanistic studies to date focus exclusively on males. Key questions involve the following: Why are males more resilient than females to certain types of stress paradigms, but not others? Are the molecular mechanisms promoting resilience sex-specific? This is likely, given the results of a recent large-scale RNA-seq study of multiple brain regions in depressed men and women, and in chronically stressed male and female mice, which demonstrated dramatic sex differences in gene expression abnormalities associated with depression and stress (Labonte et al., 2017). - Most of the studies described in this review were performed on microdissection of bulk tissue. However, the brain is an extremely heterogeneous organ composed of multiple cell types with very different cellular functions. Methodological advances such as ribosome affinity purification (TRAP) (Heiman et al., 2008), neuronal fluorescence-activated cell sorting (FACS) (Cahoy et al., 2008), and single cell drop sequencing (Drop-seq) (Macosko et al., 2015) should now be utilized to explore cell typeespecific molecular alternations associated with stress resilience. - A common methodological approach used to study the role of specific genes in stress resilience is viral-mediated overexpression or knockdown of that gene within a given brain region, or cell type. However, with this approach, the targeted gene is often manipulated to a much greater in extent (up or down) than the physiological change observed in vivo. Technological advances such as modified zinc finger proteins

References

225

(Heller et al., 2014), (Hamilton et al., 2017) or CRISPR/Cas9 can now be used to mimic endogenous molecular interactions to more clearly delineate the necessity and sufficiency of particular mechanisms of resilience.

Summary The molecular basis of stress resilience is complex, with numerous brain regions and enumerable molecular mediators involved. Although resilience appears to restore controllike behavior, multiple studies point to active processes occurring at the molecular level to compensate for, bypass, or overcome the harmful effects of stress (Fig. 15.4). Such unique mechanisms of stress resilience are the product of interactions between genetics and the environment. Delineating the complex interactions between the genetic, epigenetic, and environmental factors promoting stress resilience is essential to understand this complex phenotype and can bring us one step closer to leveraging these molecular mechanisms to develop better ways to prevent and treat MDD and other stress-related conditions.

References Akil, H., Gordon, J., Hen, R., Javitch, J., Mayberg, H., McEwen, B., et al., 2017. Treatment resistant depression: a multiscale, systems biology approach [Review] Neuroscience and Biobehavioral Reviews. https://doi.org/10.1016/ j.neubiorev.2017.08.019. Bagot, R.C., Cates, H.M., Purushothaman, I., Lorsch, Z.S., Walker, D.M., Wang, J., et al., 2016. Circuit-wide transcriptional profiling reveals brain region-specific gene networks regulating depression susceptibility. Neuron 90 (5), 969e983. https://doi.org/10.1016/j.neuron.2016.04.015. Bagot, R.C., Cates, H.M., Purushothaman, I., Vialou, V., Heller, E.A., Yieh, L., et al., 2017. Ketamine and imipramine reverse transcriptional signatures of susceptibility and induce resilience-specific gene expression profiles. Biological Psychiatry 81 (4), 285e295. https://doi.org/10.1016/j.biopsych.2016.06.012. Bagot, R.C., Parise, E.M., Pena, C.J., Zhang, H.X., Maze, I., Chaudhury, D., et al., 2015. Ventral hippocampal afferents to the nucleus accumbens regulate susceptibility to depression [Research Support, N.I.H., ExtramuralResearch Support, Non-U.S. Gov’t] Nature Communications 6, 7062. https://doi.org/10.1038/ncomms8062. Bale, T.L., Baram, T.Z., Brown, A.S., Goldstein, J.M., Insel, T.R., McCarthy, M.M., et al., 2010. Early life programming and neurodevelopmental disorders [Research Support, N.I.H., Extramural Research Support, Non-U.S. Gov’t Review] Biological Psychiatry 68 (4), 314e319. https://doi.org/10.1016/j.biopsych.2010.05.028. Benatti, C., Valensisi, C., Blom, J.M., Alboni, S., Montanari, C., Ferrari, F., et al., 2012. Transcriptional profiles underlying vulnerability and resilience in rats exposed to an acute unavoidable stress [Research Support, Non-U.S. Gov’t] Journal of Neuroscience Research 90 (11), 2103e2115. https://doi.org/10.1002/jnr.23100. Bergstrom, A., Jayatissa, M.N., Thykjaer, T., Wiborg, O., 2007. Molecular pathways associated with stress resilience and drug resistance in the chronic mild stress rat model of depression: a gene expression study [Research Support, Non-U.S. Gov’t] Journal of Molecular Neuroscience 33 (2), 201e215. Berton, O., Covington 3rd, H.E., Ebner, K., Tsankova, N.M., Carle, T.L., Ulery, P., et al., 2007. Induction of deltaFosB in the periaqueductal gray by stress promotes active coping responses [Research Support, N.I.H., Extramural Research Support, Non-U.S. Gov’t] Neuron 55 (2), 289e300. https://doi.org/10.1016/j.neuron.2007.06.033. Berton, O., McClung, C.A., Dileone, R.J., Krishnan, V., Renthal, W., Russo, S.J., et al., 2006. Essential role of BDNF in the mesolimbic dopamine pathway in social defeat stress [Research Support, Non-U.S. Gov’t] Science 311 (5762), 864e868. https://doi.org/10.1126/science.1120972. Brachman, R.A., Lehmann, M.L., Maric, D., Herkenham, M., 2015. Lymphocytes from chronically stressed mice confer antidepressant-like effects to naive mice [Research Support, N.I.H., Intramural Research Support, NonU.S. Gov’t] The Journal of Neuroscience: The Official Journal of the Society for Neuroscience 35 (4), 1530e1538. https://doi.org/10.1523/JNEUROSCI.2278-14.2015.

226

15. Molecular characterization of the resilient brain: transcriptional and epigenetic mechanisms

Bruchas, M.R., Schindler, A.G., Shankar, H., Messinger, D.I., Miyatake, M., Land, B.B., et al., 2011. Selective p38alpha MAPK deletion in serotonergic neurons produces stress resilience in models of depression and addiction [Research Support, N.I.H., Extramural Research Support, Non-U.S. Gov’t] Neuron 71 (3), 498e511. https:// doi.org/10.1016/j.neuron.2011.06.011. Cahoy, J.D., Emery, B., Kaushal, A., Foo, L.C., Zamanian, J.L., Christopherson, K.S., et al., 2008. A transcriptome database for astrocytes, neurons, and oligodendrocytes: a new resource for understanding brain development and function [Research Support, N.I.H., Extramural Research Support, Non-U.S. Gov’t] The Journal of Neuroscience: The Official Journal of the Society for Neuroscience 28 (1), 264e278. https://doi.org/10.1523/JNEUROSCI.417807.2008. Cao, J.L., Covington 3rd, H.E., Friedman, A.K., Wilkinson, M.B., Walsh, J.J., Cooper, D.C., et al., 2010. Mesolimbic dopamine neurons in the brain reward circuit mediate susceptibility to social defeat and antidepressant action. The Journal of Neuroscience: The Official Journal of the Society for Neuroscience 30 (49), 16453e16458. https://doi.org/10.1523/JNEUROSCI.3177-10.2010. Chaudhury, D., Walsh, J.J., Friedman, A.K., Juarez, B., Ku, S.M., Koo, J.W., et al., 2013. Rapid regulation of depression-related behaviours by control of midbrain dopamine neurons [Research Support, N.I.H., ExtramuralResearch Support, Non-U.S. Gov’t] Nature 493 (7433), 532e536. https://doi.org/10.1038/ nature11713. Chaudhury, S., Aurbach, E.L., Sharma, V., Blandino Jr., P., Turner, C.A., Watson, S.J., Akil, H., 2014. FGF2 is a target and a trigger of epigenetic mechanisms associated with differences in emotionality: partnership with H3K9me3 [Research Support, N.I.H., Extramural Research Support, Non-U.S. Gov’t Research Support, U.S. Gov’t, NonP.H.S.] Proceedings of the National Academy of Sciences of the United States of America 111 (32), 11834e11839. https://doi.org/10.1073/pnas.1411618111. Christoffel, D.J., Golden, S.A., Dumitriu, D., Robison, A.J., Janssen, W.G., Ahn, H.F., et al., 2011. IkappaB kinase regulates social defeat stress-induced synaptic and behavioral plasticity [Research Support, Non-U.S. Gov’t] The Journal of Neuroscience: The Official Journal of the Society for Neuroscience 31 (1), 314e321. https://doi.org/ 10.1523/JNEUROSCI.4763-10.2011. Christoffel, D.J., Golden, S.A., Heshmati, M., Graham, A., Birnbaum, S., Neve, R.L., et al., 2012. Effects of inhibitor of kappaB kinase activity in the nucleus accumbens on emotional behavior [Research Support, N.I.H., Extramural Research Support, Non-U.S. Gov’t] Neuropsychopharmacology: Official Publication of the American College of Neuropsychopharmacology 37 (12), 2615e2623. https://doi.org/10.1038/npp.2012.121. CONVERGE Consortium, 2015. Sparse whole-genome sequencing identifies two loci for major depressive disorder, [Research Support, N.I.H., Extramural Research Support, Non-U.S. Gov’t]. Nature 523 (7562), 588e591. https://doi.org/10.1038/nature14659. Couch, Y., Anthony, D.C., Dolgov, O., Revischin, A., Festoff, B., Santos, A.I., et al., 2013. Microglial activation, increased TNF and SERT expression in the prefrontal cortex define stress-altered behaviour in mice susceptible to anhedonia [Research Support, Non-U.S. Gov’t] Brain, Behavior, and Immunity 29, 136e146. https:// doi.org/10.1016/j.bbi.2012.12.017. Dias, C., Feng, J., Sun, H., Shao, N.Y., Mazei-Robison, M.S., Damez-Werno, D., et al., 2014. beta-catenin mediates stress resilience through Dicer1/microRNA regulation [Research Support, N.I.H., Extramural Research Support, Non-U.S. Gov’t] Nature 516 (7529), 51e55. https://doi.org/10.1038/nature13976. Donahue, R.J., Muschamp, J.W., Russo, S.J., Nestler, E.J., Carlezon Jr., W.A., 2014. Effects of striatal DeltaFosB overexpression and ketamine on social defeat stress-induced anhedonia in mice [Research Support, N.I.H., Extramural] Biological Psychiatry 76 (7), 550e558. https://doi.org/10.1016/j.biopsych.2013.12.014. Duclot, F., Kabbaj, M., 2013. Individual differences in novelty seeking predict subsequent vulnerability to social defeat through a differential epigenetic regulation of brain-derived neurotrophic factor expression [Research Support, N.I.H., Extramural Research Support, Non-U.S. Gov’t] The Journal of Neuroscience: The Official Journal of the Society for Neuroscience 33 (27), 11048e11060. https://doi.org/10.1523/JNEUROSCI.0199-13.2013. Duman, R.S., 2014. Pathophysiology of depression and innovative treatments: remodeling glutamatergic synaptic connections [Research Support, N.I.H., Extramural Review] Dialogues in Clinical Neuroscience 16 (1), 11e27. Duman, R.S., Heninger, G.R., Nestler, E.J., 1997. A molecular and cellular theory of depression [Research Support, Non-U.S. Gov’t Research Support, U.S. Gov’t, Non-P.H.S. Research Support, U.S. Gov’t, P.H.S. Review] Archives of General Psychiatry 54 (7), 597e606.

References

227

Elliott, E., Ezra-Nevo, G., Regev, L., Neufeld-Cohen, A., Chen, A., 2010. Resilience to social stress coincides with functional DNA methylation of the Crf gene in adult mice [Research Support, Non-U.S. Gov’t] Nature Neuroscience 13 (11), 1351e1353. https://doi.org/10.1038/nn.2642. Espallergues, J., Teegarden, S.L., Veerakumar, A., Boulden, J., Challis, C., Jochems, J., et al., 2012. HDAC6 regulates glucocorticoid receptor signaling in serotonin pathways with critical impact on stress resilience [Research Support, N.I.H., Extramural Research Support, Non-U.S. Gov’t] The Journal of Neuroscience: The Official Journal of the Society for Neuroscience 32 (13), 4400e4416. https://doi.org/10.1523/JNEUROSCI.5634-11.2012. Feder, A., Nestler, E.J., Charney, D.S., 2009. Psychobiology and molecular genetics of resilience [Review] Nature Reviews Neuroscience 10 (6), 446e457. https://doi.org/10.1038/nrn2649. Felitti, V.J., Anda, R.F., Nordenberg, D., Williamson, D.F., Spitz, A.M., Edwards, V., et al., 1998. Relationship of childhood abuse and household dysfunction to many of the leading causes of death in adults. The adverse childhood experiences (ACE) study [Research Support, U.S. Gov’t, P.H.S.] American Journal of Preventive Medicine 14 (4), 245e258. Feng, J., Pena, C.J., Purushothaman, I., Engmann, O., Walker, D., Brown, A.N., et al., 2017. Tet1 in nucleus accumbens opposes depression- and anxiety-like behaviors. Neuropsychopharmacology: Official Publication of the American College of Neuropsychopharmacology 42 (8), 1657e1669. https://doi.org/10.1038/npp.2017.6. Francis, D.D., Champagne, F.A., Liu, D., Meaney, M.J., 1999. Maternal care, gene expression, and the development of individual differences in stress reactivity [Review] Annals of the New York Academy of Sciences 896, 66e84. Franklin, T.B., Saab, B.J., Mansuy, I.M., 2012. Neural mechanisms of stress resilience and vulnerability [Research Support, Non-U.S. Gov’t Review] Neuron 75 (5), 747e761. https://doi.org/10.1016/j.neuron.2012.08.016. Friedman, A.K., Walsh, J.J., Juarez, B., Ku, S.M., Chaudhury, D., Wang, J., et al., 2014. Enhancing depression mechanisms in midbrain dopamine neurons achieves homeostatic resilience [Research Support, N.I.H., Extramural Research Support, Non-U.S. Gov’t] Science 344 (6181), 313e319. https://doi.org/10.1126/science.1249240. Golden, S.A., Covington 3rd, H.E., Berton, O., Russo, S.J., 2011. A standardized protocol for repeated social defeat stress in mice [Research Support, N.I.H., Extramural] Nature Protocols 6 (8), 1183e1191. https://doi.org/ 10.1038/nprot.2011.361. Hamilton, D.E., Cooke, C.L., Carter, B.S., Akil, H., Watson, S.J., Thompson, R.C., 2014. Basal microRNA expression patterns in reward circuitry of selectively bred high-responder and low-responder rats vary by brain region and genotype [Research Support, N.I.H., Extramural] Physiological Genomics 46 (8), 290e301. https://doi.org/ 10.1152/physiolgenomics.00152.2013. Hamilton, P.J., Burek, D.J., Lombroso, S.I., Neve, R.L., Robison, A.J., Nestler, E.J., Heller, E.A., 2017. Cell-type-specific epigenetic editing at the Fosb gene controls susceptibility to social defeat stress. Neuropsychopharmacology: Official Publication of the American College of Neuropsychopharmacology. https://doi.org/10.1038/npp.2017.88. Hammels, C., Prickaerts, J., Kenis, G., Vanmierlo, T., Fischer, M., Steinbusch, H.W., et al., 2015. Differential susceptibility to chronic social defeat stress relates to the number of Dnmt3a-immunoreactive neurons in the hippocampal dentate gyrus [Research Support, Non-U.S. Gov’t] Psychoneuroendocrinology 51, 547e556. https://doi.org/ 10.1016/j.psyneuen.2014.09.021. Harris, A.Z., Atsak, P., Bretton, Z.H., Holt, E.S., Alam, R., Morton, M.P., et al., 2017. A novel method for chronic social defeat stress in female mice. Neuropsychopharmacology : Official Publication of the American College of Neuropsychopharmacology. https://doi.org/10.1038/npp.2017.259. Heim, C., Newport, D.J., Mletzko, T., Miller, A.H., Nemeroff, C.B., 2008. The link between childhood trauma and depression: insights from HPA axis studies in humans [Research Support, N.I.H., Extramural Research Support, Non-U.S. Gov’t Research Support, U.S. Gov’t, Non-P.H.S. Review] Psychoneuroendocrinology 33 (6), 693e710. https://doi.org/10.1016/j.psyneuen.2008.03.008. Heiman, M., Schaefer, A., Gong, S., Peterson, J.D., Day, M., Ramsey, K.E., et al., 2008. A translational profiling approach for the molecular characterization of CNS cell types [Research Support, N.I.H., Extramural Research Support, Non-U.S. Gov’t] Cell 135 (4), 738e748. https://doi.org/10.1016/j.cell.2008.10.028. Heller, E.A., Cates, H.M., Pena, C.J., Sun, H., Shao, N., Feng, J., et al., 2014. Locus-specific epigenetic remodeling controls addiction- and depression-related behaviors [Research Support, N.I.H., Extramural Research Support, NonU.S. Gov’t] Nature Neuroscience 17 (12), 1720e1727. https://doi.org/10.1038/nn.3871. Heller, E.A., Hamilton, P.J., Burek, D.D., Lombroso, S.I., Pena, C.J., Neve, R.L., Nestler, E.J., 2016. Targeted epigenetic remodeling of the Cdk5 gene in nucleus accumbens regulates cocaine- and stress-evoked behavior [Research Support, Non-U.S. Gov’t Research Support, N.I.H., Extramural] Journal of Neuroscience: The Official Journal of the Society for Neuroscience 36 (17), 4690e4697. https://doi.org/10.1523/JNEUROSCI.0013-16.2016.

228

15. Molecular characterization of the resilient brain: transcriptional and epigenetic mechanisms

Hellwig, S., Brioschi, S., Dieni, S., Frings, L., Masuch, A., Blank, T., Biber, K., 2016. Altered microglia morphology and higher resilience to stress-induced depression-like behavior in CX3CR1-deficient mice. Brain, Behavior, and Immunity 55, 126e137. https://doi.org/10.1016/j.bbi.2015.11.008. Henningsen, K., Palmfeldt, J., Christiansen, S., Baiges, I., Bak, S., Jensen, O.N., et al., 2012. Candidate hippocampal biomarkers of susceptibility and resilience to stress in a rat model of depression [Research Support, Non-U.S. Gov’t] Molecular and Cellular Proteomics 11 (7), 016428. https://doi.org/10.1074/mcp.M111.016428. M111. Hetzel, A., Rosenkranz, J.A., 2014. Distinct effects of repeated restraint stress on basolateral amygdala neuronal membrane properties in resilient adolescent and adult rats [Research Support, N.I.H., Extramural] Neuropsychopharmacology: Official Publication of the American College of Neuropsychopharmacology 39 (9), 2114e2130. https:// doi.org/10.1038/npp.2014.60. Higuchi, F., Uchida, S., Yamagata, H., Abe-Higuchi, N., Hobara, T., Hara, K., et al., 2016. Hippocampal MicroRNA124 enhances chronic stress resilience in mice. The Journal of Neuroscience: The Official Journal of the Society for Neuroscience 36 (27), 7253e7267. https://doi.org/10.1523/JNEUROSCI.0319-16.2016. Hill, M.N., Hellemans, K.G., Verma, P., Gorzalka, B.B., Weinberg, J., 2012. Neurobiology of chronic mild stress: parallels to major depression [Research Support, N.I.H., Extramural Research Support, Non-U.S. Gov’t Review] Neuroscience and Biobehavioral Reviews 36 (9), 2085e2117. https://doi.org/10.1016/j.neubiorev.2012.07.001. Hodes, G.E., Pfau, M.L., Leboeuf, M., Golden, S.A., Christoffel, D.J., Bregman, D., et al., 2014. Individual differences in the peripheral immune system promote resilience versus susceptibility to social stress [Research Support, N.I.H., Extramural Research Support, Non-U.S. Gov’t] Proceedings of the National Academy of Sciences of the United States of America 111 (45), 16136e16141. https://doi.org/10.1073/pnas.1415191111. Hodes, G.E., Pfau, M.L., Purushothaman, I., Ahn, H.F., Golden, S.A., Christoffel, D.J., et al., 2015. Sex differences in nucleus accumbens transcriptome profiles associated with susceptibility versus resilience to subchronic variable stress [Research Support, N.I.H., Extramural Research Support, Non-U.S. Gov’t] The Journal of Neuroscience: The Official Journal of the Society for Neuroscience 35 (50), 16362e16376. https://doi.org/10.1523/JNEUROSCI.1392-15.2015. Hodes, G.E., Walker, D.M., Labonte, B., Nestler, E.J., Russo, S.J., 2017. Understanding the epigenetic basis of sex differences in depression [Review] Journal of Neuroscience Research 95 (1e2), 692e702. https://doi.org/10.1002/ jnr.23876. Hyde, C.L., Nagle, M.W., Tian, C., Chen, X., Paciga, S.A., Wendland, J.R., et al., 2016. Identification of 15 genetic loci associated with risk of major depression in individuals of European descent [Comparative Study] Nature Genetics 48 (9), 1031e1036. https://doi.org/10.1038/ng.3623. Issler, O., Chen, A., 2015. Determining the role of microRNAs in psychiatric disorders [Review] Nature Reviews Neuroscience 16 (4), 201e212. https://doi.org/10.1038/nrn3879. Issler, O., Haramati, S., Paul, E.D., Maeno, H., Navon, I., Zwang, R., et al., 2014. MicroRNA 135 is essential for chronic stress resiliency, antidepressant efficacy, and intact serotonergic activity [Research Support, N.I.H., Extramural Research Support, Non-U.S. Gov’t] Neuron 83 (2), 344e360. https://doi.org/10.1016/j.neuron.2014.05.042. Jiang, Y., Loh, Y.E., Rajarajan, P., Hirayama, T., Liao, W., Kassim, B.S., et al., 2017. The methyltransferase SETDB1 regulates a large neuron-specific topological chromatin domain. Nature Genetics 49 (8), 1239e1250. https:// doi.org/10.1038/ng.3906. Jochems, J., Teegarden, S.L., Chen, Y., Boulden, J., Challis, C., Ben-Dor, G.A., et al., 2015. Enhancement of stress resilience through histone deacetylase 6-mediated regulation of glucocorticoid receptor chaperone dynamics [Research Support, N.I.H., Extramural Research Support, Non-U.S. Gov’t] Biological Psychiatry 77 (4), 345e355. https://doi.org/10.1016/j.biopsych.2014.07.036. Kenworthy, C.A., Sengupta, A., Luz, S.M., Ver Hoeve, E.S., Meda, K., Bhatnagar, S., Abel, T., 2014. Social defeat induces changes in histone acetylation and expression of histone modifying enzymes in the ventral hippocampus, prefrontal cortex, and dorsal raphe nucleus [Research Support, U.S. Gov’t, Non-P.H.S.] Neuroscience 264, 88e98. https://doi.org/10.1016/j.neuroscience.2013.01.024. Korosi, A., Shanabrough, M., McClelland, S., Liu, Z.W., Borok, E., Gao, X.B., et al., 2010. Early-life experience reduces excitation to stress-responsive hypothalamic neurons and reprograms the expression of corticotropin-releasing hormone [Research Support, N.I.H., Extramural] The Journal of Neuroscience: The Official Journal of the Society for Neuroscience 30 (2), 703e713. https://doi.org/10.1523/JNEUROSCI.4214-09.2010. Krishnan, V., Han, M.H., Graham, D.L., Berton, O., Renthal, W., Russo, S.J., et al., 2007. Molecular adaptations underlying susceptibility and resistance to social defeat in brain reward regions [Research Support, N.I.H., Extramural Research Support, Non-U.S. Gov’t] Cell 131 (2), 391e404. https://doi.org/10.1016/j.cell.2007.09.018.

References

229

Labonte, B., Engmann, O., Purushothaman, I., Menard, C., Wang, J., Tan, C., et al., 2017. Sex-specific transcriptional signatures in human depression. Nature Medicine 23 (9), 1102e1111. https://doi.org/10.1038/nm.4386. Lepack, A.E., Bagot, R.C., Pena, C.J., Loh, Y.E., Farrelly, L.A., Lu, Y., et al., 2016. Aberrant H3.3 dynamics in NAc promote vulnerability to depressive-like behavior. Proceedings of the National Academy of Sciences of the United States of America 113 (44), 12562e12567. https://doi.org/10.1073/pnas.1608270113. Li, M.X., Zheng, H.L., Luo, Y., He, J.G., Wang, W., Han, J., et al., 2017. Gene deficiency and pharmacological inhibition of caspase-1 confers resilience to chronic social defeat stress via regulating the stability of surface AMPARs. Molecular Psychiatry. https://doi.org/10.1038/mp.2017.76. Liu, D., Diorio, J., Tannenbaum, B., Caldji, C., Francis, D., Freedman, A., et al., 1997. Maternal care, hippocampal glucocorticoid receptors, and hypothalamic-pituitary-adrenal responses to stress [Research Support, Non-U.S. Gov’t] Science 277 (5332), 1659e1662. Lobo, M.K., Zaman, S., Damez-Werno, D.M., Koo, J.W., Bagot, R.C., DiNieri, J.A., et al., 2013. DeltaFosB induction in striatal medium spiny neuron subtypes in response to chronic pharmacological, emotional, and optogenetic stimuli. The Journal of Neuroscience: The Official Journal of the Society for Neuroscience 33 (47), 18381e18395. https://doi.org/10.1523/JNEUROSCI.1875-13.2013. Macosko, E.Z., Basu, A., Satija, R., Nemesh, J., Shekhar, K., Goldman, M., et al., 2015. Highly parallel genome-wide expression profiling of individual cells using nanoliter droplets [Research Support, N.I.H., Extramural Research Support, Non-U.S. Gov’t Research Support, U.S. Gov’t, Non-P.H.S.] Cell 161 (5), 1202e1214. https://doi.org/ 10.1016/j.cell.2015.05.002. Maier, S.F., 1984. Learned helplessness and animal models of depression [Research Support, U.S. Gov’t, Non-P.H.S. Research Support, U.S. Gov’t, P.H.S. Review] Progress in Neuro-Psychopharmacology and Biological Psychiatry 8 (3), 435e446. Meaney, M.J., Szyf, M., 2005. Environmental programming of stress responses through DNA methylation: life at the interface between a dynamic environment and a fixed genome [Review] Dialogues in Clinical Neuroscience 7 (2), 103e123. Milior, G., Lecours, C., Samson, L., Bisht, K., Poggini, S., Pagani, F., et al., 2016. Fractalkine receptor deficiency impairs microglial and neuronal responsiveness to chronic stress. Brain, Behavior, and Immunity 55, 114e125. https://doi.org/10.1016/j.bbi.2015.07.024. Monteggia, L.M., Barrot, M., Powell, C.M., Berton, O., Galanis, V., Gemelli, T., et al., 2004. Essential role of brainderived neurotrophic factor in adult hippocampal function [Research Support, Non-U.S. Gov’t Research Support, U.S. Gov’t, P.H.S.] Proceedings of the National Academy of Sciences of the United States of America 101 (29), 10827e10832. https://doi.org/10.1073/pnas.0402141101. Monteggia, L.M., Luikart, B., Barrot, M., Theobold, D., Malkovska, I., Nef, S., et al., 2007. Brain-derived neurotrophic factor conditional knockouts show gender differences in depression-related behaviors [Research Support, N.I.H., Extramural Research Support, Non-U.S. Gov’t] Biological Psychiatry 61 (2), 187e197. https://doi.org/10.1016/ j.biopsych.2006.03.021. Mozhui, K., Karlsson, R.M., Kash, T.L., Ihne, J., Norcross, M., Patel, S., et al., 2010. Strain differences in stress responsivity are associated with divergent amygdala gene expression and glutamate-mediated neuronal excitability [Research Support, N.I.H., Extramural Research Support, N.I.H., Intramural] The Journal of Neuroscience: The Official Journal of the Society for Neuroscience 30 (15), 5357e5367. https://doi.org/10.1523/JNEUROSCI.501709.2010. Muller, I., Obata, K., Richter-Levin, G., Stork, O., 2014. GAD65 haplodeficiency conveys resilience in animal models of stress-induced psychopathology. Frontiers in Behavioral Neuroscience 8, 265. https://doi.org/10.3389/ fnbeh.2014.00265. Nasca, C., Bigio, B., Zelli, D., de Angelis, P., Lau, T., Okamoto, M., et al., 2017. Role of the astroglial glutamate exchanger xCT in ventral Hippocampus in resilience to stress. Neuron 96 (2), 402e413. https://doi.org/ 10.1016/j.neuron.2017.09.020 e405. Nasca, C., Bigio, B., Zelli, D., Nicoletti, F., McEwen, B.S., 2015. Mind the gap: glucocorticoids modulate hippocampal glutamate tone underlying individual differences in stress susceptibility [Research Support, N.I.H., Extramural Research Support, Non-U.S. Gov’t] Molecular Psychiatry 20 (6), 755e763. https://doi.org/10.1038/mp.2014.96. Nestler, E.J., 2014. Epigenetic mechanisms of depression [Review] JAMA Psychiatry 71 (4), 454e456. https://doi.org/ 10.1001/jamapsychiatry.2013.4291.

230

15. Molecular characterization of the resilient brain: transcriptional and epigenetic mechanisms

Nestler, E.J., 2015. FosB: a transcriptional regulator of stress and antidepressant responses [Research Support, N.I.H., Extramural Review] European Journal of Pharmacology 753, 66e72. https://doi.org/10.1016/j.ejphar.2014.10.034. Nestler, E.J., Hyman, S.E., 2010. Animal models of neuropsychiatric disorders [Review] Nature Neuroscience 13 (10), 1161e1169. https://doi.org/10.1038/nn.2647. Ohnishi, Y.N., Ohnishi, Y.H., Vialou, V., Mouzon, E., LaPlant, Q., Nishi, A., Nestler, E.J., 2015. Functional role of the N-terminal domain of DeltaFosB in response to stress and drugs of abuse [Research Support, N.I.H., Extramural Research Support, Non-U.S. Gov’t] Neuroscience 284, 165e170. https://doi.org/10.1016/ j.neuroscience.2014.10.002. Ota, K.T., Liu, R.J., Voleti, B., Maldonado-Aviles, J.G., Duric, V., Iwata, M., et al., 2014. REDD1 is essential for stressinduced synaptic loss and depressive behavior [Research Support, N.I.H., Extramural Research Support, Non-U.S. Gov’t] Nature Medicine 20 (5), 531e535. https://doi.org/10.1038/nm.3513. Palmfeldt, J., Henningsen, K., Eriksen, S.A., Muller, H.K., Wiborg, O., 2016. Protein biomarkers of susceptibility and resilience to stress in a rat model of depression. Molecular and Cellular Neurosciences 74, 87e95. https://doi.org/ 10.1016/j.mcn.2016.04.001. Pearson-Leary, J., Eacret, D., Chen, R., Takano, H., Nicholas, B., Bhatnagar, S., 2017. Inflammation and vascular remodeling in the ventral hippocampus contributes to vulnerability to stress. Translational Psychiatry 7 (6), e1160. https://doi.org/10.1038/tp.2017.122. Pena, C.J., Kronman, H.G., Walker, D.M., Cates, H.M., Bagot, R.C., Purushothaman, I., et al., 2017. Early life stress confers lifelong stress susceptibility in mice via ventral tegmental area OTX2 [Research Support, N.I.H., Extramural] Science 356 (6343), 1185e1188. https://doi.org/10.1126/science.aan4491. Rimmerman, N., Schottlender, N., Reshef, R., Dan-Goor, N., Yirmiya, R., 2017. The hippocampal transcriptomic signature of stress resilience in mice with microglial fractalkine receptor (CX3CR1) deficiency. Brain, Behavior, and Immunity 61, 184e196. https://doi.org/10.1016/j.bbi.2016.11.023. Robison, A.J., Vialou, V., Sun, H.S., Labonte, B., Golden, S.A., Dias, C., et al., 2014. Fluoxetine epigenetically alters the CaMKIIalpha promoter in nucleus accumbens to regulate DeltaFosB binding and antidepressant effects [Research Support, N.I.H., Extramural] Neuropsychopharmacology: Official Publication of the American College of Neuropsychopharmacology 39 (5), 1178e1186. https://doi.org/10.1038/npp.2013.319. Russo, S.J., Murrough, J.W., Han, M.H., Charney, D.S., Nestler, E.J., 2012. Neurobiology of resilience [Research Support, N.I.H., Extramural Review] Nature Neuroscience 15 (11), 1475e1484. https://doi.org/10.1038/nn.3234. Schmidt, M.V., Trumbach, D., Weber, P., Wagner, K., Scharf, S.H., Liebl, C., et al., 2010. Individual stress vulnerability is predicted by short-term memory and AMPA receptor subunit ratio in the hippocampus [Research Support, Non-U.S. Gov’t] The Journal of Neuroscience: The Official Journal of the Society for Neuroscience 30 (50), 16949e16958. https://doi.org/10.1523/JNEUROSCI.4668-10.2010. Stead, J.D., Clinton, S., Neal, C., Schneider, J., Jama, A., Miller, S., et al., 2006. Selective breeding for divergence in novelty-seeking traits: heritability and enrichment in spontaneous anxiety-related behaviors [Research Support, N.I.H., Extramural Research Support, U.S. Gov’t, Non-P.H.S.] Behavior Genetics 36 (5), 697e712. https:// doi.org/10.1007/s10519-006-9058-7. Steinman, M.Q., Trainor, B.C., 2017. Sex differences in the effects of social defeat on brain and behavior in the California mouse: insights from a monogamous rodent [Review] Seminars in Cell and Developmental Biology 61, 92e98. https://doi.org/10.1016/j.semcdb.2016.06.021. Su, C.L., Su, C.W., Hsiao, Y.H., Gean, P.W., 2016. Epigenetic regulation of BDNF in the learned helplessness-induced animal model of depression [Research Support, Non-U.S. Gov’t] Journal of Psychiatric Research 76, 101e110. https://doi.org/10.1016/j.jpsychires.2016.02.008. Sullivan, P.F., Neale, M.C., Kendler, K.S., 2000. Genetic epidemiology of major depression: review and meta-analysis [Meta-Analysis Research Support, U.S. Gov’t, P.H.S.] The American Journal of Psychiatry 157 (10), 1552e1562. https://doi.org/10.1176/appi.ajp.157.10.1552. Sun, H., Damez-Werno, D.M., Scobie, K.N., Shao, N.Y., Dias, C., Rabkin, J., et al., 2015. ACF chromatin-remodeling complex mediates stress-induced depressive-like behavior [Research Support, N.I.H., Extramural Research Support, Non-U.S. Gov’t] Nature Medicine 21 (10), 1146e1153. https://doi.org/10.1038/nm.3939. Sun, H., Martin, J.A., Werner, C.T., Wang, Z.J., Damez-Werno, D.M., Scobie, K.N., et al., 2016. BAZ1B in nucleus accumbens regulates reward-related behaviors in response to distinct emotional stimuli [Research Support, N.I.H., Extramural Research Support, Non-U.S. Gov’t] The Journal of Neuroscience: The Official Journal of the Society for Neuroscience 36 (14), 3954e3961. https://doi.org/10.1523/JNEUROSCI.3254-15.2016.

References

231

Takahashi, A., Chung, J.R., Zhang, S., Zhang, H., Grossman, Y., Aleyasin, H., et al., 2017. Establishment of a repeated social defeat stress model in female mice. Scientific Reports 7 (1), 12838. https://doi.org/10.1038/s41598-01712811-8. Taliaz, D., Loya, A., Gersner, R., Haramati, S., Chen, A., Zangen, A., 2011. Resilience to chronic stress is mediated by hippocampal brain-derived neurotrophic factor [Research Support, Non-U.S. Gov’t] The Journal of Neuroscience: The Official Journal of the Society for Neuroscience 31 (12), 4475e4483. https://doi.org/10.1523/JNEUROSCI.5725-10.2011. Touma, C., Bunck, M., Glasl, L., Nussbaumer, M., Palme, R., Stein, H., et al., 2008. Mice selected for high versus low stress reactivity: a new animal model for affective disorders [Research Support, Non-U.S. Gov’t] Psychoneuroendocrinology 33 (6), 839e862. https://doi.org/10.1016/j.psyneuen.2008.03.013. Tsankova, N.M., Berton, O., Renthal, W., Kumar, A., Neve, R.L., Nestler, E.J., 2006. Sustained hippocampal chromatin regulation in a mouse model of depression and antidepressant action [Research Support, N.I.H., Extramural] Nature Neuroscience 9 (4), 519e525. https://doi.org/10.1038/nn1659. Tye, K.M., Mirzabekov, J.J., Warden, M.R., Ferenczi, E.A., Tsai, H.C., Finkelstein, J., et al., 2013. Dopamine neurons modulate neural encoding and expression of depression-related behaviour [Research Support, N.I.H., Extramural Research Support, Non-U.S. Gov’t Research Support, U.S. Gov’t, Non-P.H.S.] Nature 493 (7433), 537e541. https://doi.org/10.1038/nature11740. Uchida, S., Hara, K., Kobayashi, A., Otsuki, K., Yamagata, H., Hobara, T., et al., 2011. Epigenetic status of Gdnf in the ventral striatum determines susceptibility and adaptation to daily stressful events [Research Support, Non-U.S. Gov’t] Neuron 69 (2), 359e372. https://doi.org/10.1016/j.neuron.2010.12.023. Vialou, V., Maze, I., Renthal, W., LaPlant, Q.C., Watts, E.L., Mouzon, E., et al., 2010a. Serum response factor promotes resilience to chronic social stress through the induction of DeltaFosB [Research Support, N.I.H., Extramural Research Support, Non-U.S. Gov’t] The Journal of Neuroscience: The Official Journal of the Society for Neuroscience 30 (43), 14585e14592. https://doi.org/10.1523/JNEUROSCI.2496-10.2010. Vialou, V., Robison, A.J., Laplant, Q.C., Covington 3rd, H.E., Dietz, D.M., Ohnishi, Y.N., et al., 2010b. DeltaFosB in brain reward circuits mediates resilience to stress and antidepressant responses [Research Support, N.I.H., Extramural Research Support, Non-U.S. Gov’t] Nature Neuroscience 13 (6), 745e752. https://doi.org/10.1038/nn.2551. Vyas, A., Jadhav, S., Chattarji, S., 2006. Prolonged behavioral stress enhances synaptic connectivity in the basolateral amygdala [Research Support, Non-U.S. Gov’t] Neuroscience 143 (2), 387e393. https://doi.org/10.1016/ j.neuroscience.2006.08.003. Wilkinson, M.B., Xiao, G., Kumar, A., LaPlant, Q., Renthal, W., Sikder, D., et al., 2009. Imipramine treatment and resiliency exhibit similar chromatin regulation in the mouse nucleus accumbens in depression models [Research Support, N.I.H., Extramural Research Support, Non-U.S. Gov’t] The Journal of Neuroscience: The Official Journal of the Society for Neuroscience 29 (24), 7820e7832. https://doi.org/10.1523/JNEUROSCI.0932-09.2009. Willner, P., 1986. Validation criteria for animal models of human mental disorders: learned helplessness as a paradigm case. Progress in Neuro-psychopharmacology and Biological Psychiatry 10 (6), 677e690. Willner, P., 2005. Chronic mild stress (CMS) revisited: consistency and behavioural-neurobiological concordance in the effects of CMS [Review] Neuropsychobiology 52 (2), 90e110. https://doi.org/10.1159/000087097. Willner, P., Muscat, R., Papp, M., 1992. Chronic mild stress-induced anhedonia: a realistic animal model of depression [Research Support, Non-U.S. Gov’t Review] Neuroscience and Biobehavioral Reviews 16 (4), 525e534. Willner, P., Towell, A., Sampson, D., Sophokleous, S., Muscat, R., 1987. Reduction of sucrose preference by chronic unpredictable mild stress, and its restoration by a tricyclic antidepressant [Research Support, Non-U.S. Gov’t] Psychopharmacology 93 (3), 358e364. Yang, C.H., Huang, C.C., Hsu, K.S., 2012. A critical role for protein tyrosine phosphatase nonreceptor type 5 in determining individual susceptibility to develop stress-related cognitive and morphological changes [Research Support, Non-U.S. Gov’t] The Journal of Neuroscience: The Official Journal of the Society for Neuroscience 32 (22), 7550e7562. https://doi.org/10.1523/JNEUROSCI.5902-11.2012.

C H A P T E R

16

The role of the CRF-urocortin system in stress resilience 1

Marloes J.A.G. Henckens1, Jan M. Deussing2, Alon Chen2, 3

Department of Cognitive Neuroscience, Donders Institute for Brain, Cognition and Behaviour, Radboudumc, Nijmegen, The Netherlands; 2Department of Stress Neurobiology and Behavioral Neurogenetics, Max Planck Institute of Psychiatry, Munich, Germany; 3Department of Neurobiology, Weizmann Institute of Science, Rehovot, Israel

Introduction to the corticotropin-releasing factor/urocortin system Decades of research indicate that corticotropin-releasing factor (CRF) is an essential mediator of the behavioral, endocrine, and autonomic response to stress (Dunn and Berridge, 1990). Upon stress exposure, CRF is rapidly released from the paraventricular nucleus (PVN) of the hypothalamus into the periphery to activate the hypothalamic-pituitary-adrenal (HPA) axis by stimulating the release of adrenocorticotropic hormone from the anterior pituitary, which then triggers the synthesis and secretion of corticosteroids (cortisol in humans, corticosterone in rodents) from the adrenal gland (Vale et al., 1981). CRF release also contributes to the sympathetic response to stress by activating noradrenergic neurons in the locus coeruleus (LC) (Valentino et al., 1993)dthe principal site for brain synthesis of noradrenalinedand acting on the adrenal medulla and peripheral sympathetic response system (Brown et al., 1982). However, besides the peripherally-mediated actions of CRF, its central release also contributes directly to the necessary adaptive behavioral response to stressful challenges by inducing increased vigilance, which eventually transitions into a state of anxiety (Bale and Vale, 2004). Increased central CRF levels induced either by intracerebroventricular administration or by genetic overexpression (OE) in transgenic mice are associated with an anxiogenic phenotype, whereas the suppression of CRF signaling (e.g., by the administration of CRF antisense oligodeoxynucleotides or receptor antagonists) induces anxiolytic effects and reduces stress-induced anxiety (see Reul and Holsboer, 2002, and Bale and Vale, 2004 for excellent reviews on this work). The observation of elevated

Stress Resilience https://doi.org/10.1016/B978-0-12-813983-7.00016-1

233

Copyright © 2020 Elsevier Inc. All rights reserved.

234

16. The role of the CRF-urocortin system in stress resilience

CRF levels in the cerebrospinal fluid (CSF), increased numbers of PVN CRF-expressing neurons, as well as PVN Crf mRNA in patients suffering from stress-related psychopathology, such as major depressive disorder (MDD) (Nemeroff et al., 1984; Raadsheer et al., 1994, 1995; Hartline et al., 1996; Wang et al., 2008) or posttraumatic stress disorder (PTSD) (Bremner et al., 1997; Baker and Shalhoub-Kevorkian, 1999), has implicated CRF/UCN system abnormalities in the pathophysiology of these disorders. Supporting this association, abnormalities appear to normalize with electroconvulsive therapy (Nemeroff et al., 1991) and successful antidepressant treatment (De Bellis et al., 1993; Veith et al., 1993; Heuser et al., 1998). Notably, persistent elevations of CSF CRF concentration in symptomatically improved depressed patients are associated with early relapse of depression (Banki et al., 1992). The HPA axis hyperactivity reported in depressed patients (Ising et al., 2007; Holsboer, 2003) has further substantiated the interest in the role of the CRF/urocortin (UCN) system in susceptibility to stress-related mental disease.

The corticotropin-releasing factor/urocortin system as a critical mediator of the behavioral stress response The CRF ligand family members exert their actions by binding two G proteinecoupled receptors, CRF receptor subtype 1 (CRFR1) and 2 (CRFR2). CRF has highest affinity for CRFR1 and a >20-fold lower affinity for CRFR2, which is preferentially activated by urocortin 2 (UCN2) and 3 (UCN3), whereas urocortin 1 (UCN1) binds both receptors with equal affinity (Fig. 16.1A). The ligands all display unique, partially overlapping, expression patterns throughout the brain (Fig. 16.1B). Crfr1 mRNA is abundantly expressed in the anterior pituitary, steering HPA axis activation, but also throughout the brain, with highest levels in the cerebellum and neocortical, limbic, midbrain, and brainstem regions (Van Pett et al., 2000), whereas Crfr2 mRNA is more locally expressed and virtually confined to subcortical structures (Fig. 16.1C). The anxiogenic effects of CRF have traditionally been attributed to the activation of CRFR1; its inhibition by pharmacological means prevents the CRF-induced anxiogenic phenotype (Skutella et al., 1998; Liebsch et al., 1999; Habib et al., 2000; Zorrilla et al., 2002). Conversely, constitutive inactivation of CRFR1 in developmental knockout mice reduces anxiety-like behavior as assessed in a wide variety of behavioral tests (i.e., open field, elevated plus maze, light-dark box, or defensive withdrawal) (Smith et al., 1998; Timpl et al., 1998; Contarino et al., 1999; Muller et al., 2003). These findings, together with the observation of increased PVN CRFR1 mRNA levels in patients suffering from depression (Wang et al., 2008), suggested a causative role for CRFR1 hyperactivation in stress-related psychopathologies and fostered the development of CRFR1 antagonists as potential nextgeneration anxiolytics and antidepressants (Holsboer, 1999; Zobel et al., 2000). In contrast, CRFR2 was thought to contribute to stress coping behavior and termination of the stress response, opposing CRFR1-mediated actions and restoring homeostasis. Crfr2 knockout mice display elevated corticosterone levels in response to stress (Bale et al., 2000; Coste et al., 2000), an anxiogenic phenotype (Bale et al., 2000; Kishimoto et al., 2000), and impaired stress recovery (Issler et al., 2014). However, many findings seem to contradict these circulating views on CRFR signaling and rather suggest a higher degree of complexity with the effects of CRFR activation being brain region specific, cell type specific, and synapse specific

The corticotropin-releasing factor/urocortin system as a critical mediator of the behavioral stress response

235

FIGURE 16.1 The CRF/UCN family. Following the initial discovery of CRF, several other members of the mammalian CRF-related peptide family were identified; urocortin 1 (UCN1), UCN2 (or stresscopin-related peptide), and UCN3 (or stresscopin), and some nonmammalian peptides (Dautzenberg and Hauger, 2002). This family of CRFrelated peptides exerts its actions by binding the G proteinecoupled receptors CRFR1 and CRFR2. CRFR1 and CRFR2 are expressed in various central and peripheral tissues and are produced from distinct genes and have several splice variants (CRFR1a,b, and CRFR2a,b,g) of which several are nonfunctional (Grammatopoulos and Chrousos, 2002; Keck, 2006; Perrin and Vale, 1999). The receptors exhibit w70% sequence homology, with predominant structural differences in the ligand-binding domain, inducing distinctive ligand-binding profiles. (A) CRF is a highaffinity ligand for CRFR1. UCN1 binds with equal affinity to both receptors, whereas UCN2 and UCN3 are exclusive ligands of CRFR2. However, specificity is lost at higher concentrations of the ligand, with CRF activating CRFR2 and UCN2 acting on CRFR1. CRF-binding protein (BP) binds CRF and UCN1 with an affinity equal to or even higher than that of its receptors and therefore is an important indirect regulator of CRFR activation. (B) Schematic representation of Crf, Ucn1, Ucn2, Ucn3 mRNA expression in a sagittal section of the rodent brain. (C) Schematic representation of Crfr1, Crfr2, and Crf-BP mRNA distribution in a sagittal section of the rodent brain. Key: 7, facial nucleus; 12, hypoglossal nucleus; A1, A1 noradrenaline cells; A5, A5 noradrenaline cells; Amb, ambiguous nucleus; Arc, arcuate nucleus; BAR, Barrington’s nucleus; Basel G, basal ganglia; BLA, basolateral amygdala; BNST, bed nucleus of the stria terminalis; CeA, central amygdala; Cereb, cerebellum; CingCx, cingulate cortex; CoA, cortical amygdala; DBB, diagonal band of Broca; DMH, dorsomedial hypothalamus; EW, Edinger-Westphal nucleus; FrCx, frontal cortex; GP, globus pallidus; Hip, hippocampus; IC, inferior colliculus; IO, inferior olive; IPN, interpeduncular nucleus; LC, locus coeruleus; LDTg, laterodorsal tegmental nucleus; LS, lateral septum; LSO, lateral superior olive; MeA, medial amygdala; MePO, medial preoptic area; MGN, medial geniculate nucleus; MS, medial septum; NAc, nucleus accumbens; NI, nucleus incertus; NTS, nucleus of the solitary tract; OB, olfactory bulb; OT, olfactory tubercle; OccCx, occipital cortex; PAG, periaqueductal gray; ParCx, parietal cortex; PB, parabrachial nucleus; PFA, perifornical area; PG, pontine gray; Pir, piriform cortex; PM, premammillary nucleus of the hypothalamus; PPTg, pedunculopontine tegmental nucleus; PVN, paraventricular nucleus of the hypothalamus; R, red nucleus; RN, raphe nucleus; RTN, reticular nucleus; SC, superior colliculus; SI, substantia innominata; SN, substantia nigra; SON, supraoptic nucleus; SP5n, spinal trigeminus nucleus; SPO, superior paraolivary nucleus; VMH, ventromedial hypothalamus; VTA, ventral tegmental area.

236

16. The role of the CRF-urocortin system in stress resilience

and moreover dependent on the organism’s (stress) history (Henckens et al., 2016). This might also relate to the observation that the latest clinical trials failed to demonstrate sufficient therapeutic efficacy of CRFR1 antagonists, although it is possible that more refined treatment strategies, including patient stratification, could help to overcome the currently halted CRFR1 antagonist development (Binneman et al., 2008; Paez-Pereda et al., 2011; Murrough and Charney, 2017; Dunlop et al., 2017). The emergence of new neurobiological tools, such as conditional mutagenesis, viral manipulations, and optogenetics, has allowed site-specific investigation of CRFR activation to investigate the contribution of local CRF/UCN dysfunction in closer detail. In these studies, particular focus has been attributed to its role in the PVN, amygdala, and hippocampus. These regions are key in the initiation and control of the neuroendocrine and behavioral response to stress; they all express CRFR1 and are sources of CRF-containing neurons that are activated by stress (Koob and Heinrichs, 1999; Chen et al., 2004). The PVN, a crucial hub in the regulation of the HPA axis, is characterized by high expression levels of CRF, and also UCN1, UCN2, CRFR1, and CRFR2 mRNA expression is locally observed to some extent. Activation of PVN CRF-expressing neurons initiates the behavioral as well as the HPA axis response to stress, whereas their inhibition by negative glucocorticoid feedback importantly contributes to its termination (Liposits et al., 1987; Uht et al., 1988; Herman et al., 1990). Local CRFR1 expression in the PVN also has an anxiogenic effect, albeit only in male rats (Fan et al., 2013). In the amygdala, CRF expression is restricted to the central nucleus (CeA) and the bed nucleus of the stria terminalis (BNST). The CeA is the major output nucleus of the amygdala, controlling the expression of innate behaviors and physiological responses (LeDoux et al., 1988), and the BNST is involved in sustained states of anxiety (Davis, 1998). CRFR1 is observed in all amygdalar subnuclei, with highest levels in the basolateral nucleus (BLA), which is involved in fear learning and fear memory consolidation (Roozendaal et al., 2009), and anterior BNST, followed by the medial and central nuclei (Van Pett et al., 2000). Amygdalar CRFR1 activation has been found to increase anxiety-like behavior during social interaction (Sajdyk et al., 1999; Gehlert et al., 2005; Spiga et al., 2006), augment pain responses (Ji et al., 2013) and pain-related anxiety (Ji et al., 2007), increase inhibitory avoidance behavior (Vicentini et al., 2014), reduce feeding and increase grooming behavior (Jochman et al., 2005), and induce fear-potentiated startle, while impairing prepulse inhibition (Bijlsma et al., 2011). Moreover, CRF in the BLA contributes to stress-enhanced fear memory consolidation (Roozendaal et al., 2002) through CRFR1 binding (Hubbard et al., 2007) by interacting with the b-adrenoreceptor-cyclic AMP cascade, facilitating modulation by noradrenaline in the region (Roozendaal et al., 2008). Interestingly, the anxiolytic effects of environmental enrichment were associated with very low Crfr1 mRNA levels in the BLA, implicating amygdala CRFR1 expression as a substrate by which diverse environmental factors can modify behavior. Similar to the amygdala, hippocampal function is potentiated by local CRF signaling as well; both fear learning (Blank et al., 2002) and the retention of fear memory (Hung et al., 1992) are enhanced by hippocampal CRF. Moreover, local CRF increases defensive responses and anxiety during conditioned and unconditioned threat (Pentkowski et al., 2009). Taken together, these studies implicate the CRF/UCN system in orchestrating the stress response by regulating both behavioral and neuroendocrine reactions to an acute challenge.

The corticotropin-releasing factor/urocortin system mediates stress vulnerability caused by chronic stress exposure

237

These responses enable an organism to optimally adapt to a threatening/changing environment and benefit survival, but the maintenance of this stressed, anxiety-like state irrespective of the environment is highly maladaptive and can have severe consequences for general health. Prolonged exposure to stress, or stress exposure during critical periods in development, can induce such a state and are prominent risk factors for stress-related mental illness (de Kloet et al., 2005; Nestler et al., 2016). The behavioral, neuroendocrine, and neuroplastic consequences of both chronic stress and early-life stress (ELS) therefore provide an interesting substrate for interrogating stress resilience and susceptibility.

The corticotropin-releasing factor/urocortin system mediates stress vulnerability caused by chronic stress exposure Two important animal models of chronic stress are chronic social defeat stress and chronic variable stress. Chronic social defeat stress is a paradigm in which an intruder animal is repeatedly placed in the cage of a dominant conspecific in a manner that allows for nonlethal conflict (McLaughlin et al., 2006). Chronic variable stress involves repeated exposure to physical stressors, such as restraint, foot shock, or cold (Willner, 2005). Both of these paradigms induce a behavioral state that mimics symptoms of depression, that is, social avoidance, anhedonia, weight loss, disturbed sleep, as well as increased CRF levels and HPA axis activation (Chappell et al., 1986; Krishnan et al., 2007; Pulliam et al., 2010; Page et al., 2016; Wells et al., 2017). Moreover, these chronic stressors increase anxiety and potentiate startle responses (Pulliam et al., 2010; de Andrade et al., 2013), resembling observations in PTSD (Glover et al., 2011). Furthermore, impairments in learning and memory are observed as a consequence of chronic stress (Wang et al., 2011a) and are also observed in stress-related psychiatric disorders (Brewin et al., 2007; de Kloet et al., 2005). ELS exposuredfor example, prenatally by stress in the mother or postnatally by maternal separation or impoverished maternal care induced by the limited availability of nesting material (Rice et al., 2008)dinduces a similar behavioral phenotype (Graham et al., 2011), as well as increased CRF levels and HPA axis (re)activity in adulthood (for review, see van Bodegom et al., 2017). In terms of mechanisms, exposure to ELS as well as chronic variable stress reduces the expression of the glucocorticoid receptor (GR) in the PVN, which may mediate the upregulation of CRF expression during chronic stress (Bingham et al., 2013; Makino et al., 1995; Herman et al., 1995). However, the observation of elevated CRF in the absence of GR mRNA downregulation in some stress regimens suggests that other mechanisms may also contribute to driving PVN gene expression (Herman and Tasker, 2016). A recent study pointed toward a potential role for local CRFR1-expressing neurons in modulating activation of local CRF neurons, as they reside in close proximity of the CRF neurons and are of an apparent GABAergic phenotype (Ramot et al., 2017). CRFR1 expression in these neurons is positively regulated by glucocorticoids, generating a second mechanism for feedback inhibition. Although PVN CRFR1 is not essential for the regulation of basal anxiety and HPA axis responses to acute stress, it can modulate the response to chronic stress and contribute to the resulting increase in corticosterone levels and anxiety-like behavior (Ramot et al., 2017). Besides the potential local inhibitory regulating circuits,

238

16. The role of the CRF-urocortin system in stress resilience

activity of CRF neurons is influenced by excitatory synaptic inputs. Early-life environment is capable of inducing synaptic rewiring, modifying the number and function of excitatory synapses onto these CRF neurons, and lastingly affecting their function (Korosi et al., 2010; Gunn et al., 2013). Chronic stress and ELS-induced alterations in the (local) CRF/UCN system in the PVN thereby seem to contribute to stress sensitivity. In the amygdala, chronic stress exposure and ELS induce dendritic growth (Vyas et al., 2002, 2003; Henckens et al., 2015), increase spine density (Suvrathan et al., 2014), enhance amygdala excitability (Rau et al., 2015) and long-term potentiation (Suvrathan et al., 2014), and exaggerate amygdala activation in both safe and fearful contexts (Hoffman et al., 2014), while contributing to an anxiety-like behavioral phenotype (McEwen, 2012). Research investigating the effects of (sub)chronic CRF/UCN or CRFR1 antagonist infusion into the distinct amygdalar subnuclei has revealed a prominent role of the CRF/UCN system in mediating these amygdala-controlled behavioral effects. CRF knockdown (KD) or local blockage of CRFR1 signaling in the CeA was observed to reduce stress-induced anxiety (Regev et al., 2012; Liebsch et al., 1995), suppresse chronic pain-induced anxiety (Ji et al., 2007), and prevent stress-induced hyperalgesia (Itoga et al., 2016), whereas CRF-OE in the CeA increased anxiety- and depressive-like behavior (Keen-Rhinehart et al., 2009). CRFR1 inhibition in the BNST was found to reduce chronic stresseinduced anxiety, hyperalgesia, and HPA axis activation (Tran et al., 2014), whereas CRF-OE in the BNST induced depressive-like behavior and was associated with increased Crfr1 mRNA expression levels (Regev et al., 2011). In the BLA, specific KD of CRFR1 was shown to decrease anxiety levels (Sztainberg et al., 2010), whereas repeated administration of UCN1 in the BLA inducde a persistent state of anxiety- or panic-like symptoms in the rat (Sajdyk et al., 1999; Shekhar et al., 2003; Rainnie et al., 2004), which was associated with a hyperexcitable BLA network that was NMDA receptor dependent and calcium calmodulinedependent protein kinase II (CaMKII) dependent (Rainnie et al., 2004). Interestingly, previous chronic stress exposure changed the response of the amygdala to a new CRF challenge. The CRF-induced potentiation of afferent activation of the BLA as observed in stress-naïve animals was reduced in rats with a history of chronic stress, and this reduction predicted the development of depressive-like symptoms as a consequence of the stress procedure (Sandi et al., 2008). Further analyses revealed that this reduction was greatest in highly anxious rats. These rats displayed increased amygdalar Crfr1 mRNA levels and CRF-mediated potentiation prior to the stress procedure, which were both significantly reduced following stress. The administration of a CRFR1 antagonist prevented the attenuated CRF response. The reduced acute response to CRF as a consequence of chronic stress was previously linked to a depressive-like phenotype, as this response in stress-naïve animals typically reduces behavioral despair as displayed in the tail suspension and forced swim test (Swiergiel et al., 2008). Interestingly, treatment with a CRFR1 antagonist was most effective in the highly anxious rats, which is in line with other reports (Lancel et al., 2002; Heinrichs and Koob, 2004; Keck et al., 2005), implicating that anxious patients (or those with a history of stress) displaying sensitive CRFR1 signaling might actually benefit most from treatment with CRFR1 antagonists (Hauger et al., 2006; Sanders and Nemeroff, 2016). This association might also be modulated by differential genetic background, influencing the CRF/UCN system’s sensitivity to stress (Anisman et al., 2007).

Corticotropin-releasing factor/urocortin system mechanisms influencing resilience

239

Hippocampal function is impaired by chronic stress, which is reflected in learning and memory deficits, depressed hippocampal synaptic transmission, blocked activity-induced polymerization of spine actin, and impaired synaptic plasticity in hippocampal slices (for review, see Chen et al., 2013). Many of these effects have been attributed to CRF, as chronic CRF exposure was found to deplete thin dendritic spines (i.e., the so-called “learning spines”) in the hippocampus through destabilization, resulting in a reduction of small, potentiation-ready excitatory synapses (Chen et al., 2013). This reduction in dendritic spine density was dependent on CRFR1-induced activation of NMDA receptors, which recruit the calcium-dependent enzyme calpain, triggering the breakdown of spine actin-interacting proteins (Andres et al., 2013) and on the CRFR1-mediated reduction of the hippocampal cell adhesion molecule nectin-3 (Wang et al., 2013). The involvement of CRFR1 in mediating the behavioral consequences of chronic stress is further substantiated by a greatly suppressed chronic stresseinduced phenotype in conditional forebrain CRFR1 knockout (CRFR1-CKO) mice. Mice with forebrain CRFR1 deficiency showed much milder memory impairments and normal hippocampal dendritic morphology and nectin levels as a consequence of chronic stress than their wild-type counterparts (Wang et al., 2011a). Similarly, part of the ELS-induced phenotype was dependent on CRFR1 signaling; conditional CRFR1 deletion or CRFR1 antagonist treatment prevented the typical reduced body weight gain during development and adulthood and attenuated the anxiogenic effects of ELS. In addition, the forebrain-restricted CRFR1 deficiency restored cognitive function, hippocampal long-term potentiation, and spine density (Ivy et al., 2010; Wang et al., 2011b). Along these lines, the impairments in learning and memory were mimicked by postnatal forebrain-specific CRF-OE (Wang et al., 2011b). Importantly, acute trauma (i.e., severe stress) seems to induce a similar cognitive impairment and associated decrease in hippocampal neuronal excitability, which were prevented by repeated administration of a selective CRFR1 antagonist (Philbert et al., 2013). Overall, chronic stress potently affects the CRF/UCN system and thereby increases an individual’s vulnerability to develop stress-related psychopathology. However, significant interindividual differences exist in one’s sensitivity to stress and its potency to longlastingly alter brain function. Aforementioned effects of stress exposure are not observed in all individuals, but subgroups of resilient versus vulnerable animals can be identified and characterized by apparent differential sensitivity to stress. What are the differences in CRF/UCN system function and response to stress between these subgroups? And how are these established?

Corticotropin-releasing factor/urocortin system mechanisms influencing resilience Corticotropin-releasing factor system genetic variance x environment interactions Influential studies in monozygotic twins have demonstrated that stress vulnerability can be explained partially (30%e40%) by genetic variation, mainly mediated by single-nucleotide polymorphisms (SNPs) (Afifi et al., 2010; Pitman et al., 2012). In a search for such gene x

240

16. The role of the CRF-urocortin system in stress resilience

environment (G x E) interactions for the CRF/UCN system, Bradley et al. (2008) investigated the interaction between 15 genetic polymorphisms in the Crfr1 gene and measures of childhood abuse on adult depressive symptomatology. They identified significant G x E interactions for several individual SNPs, as well as with a common haplotype spanning intron 1 of the Crfr1 locus that modified adult risk for MDD in the presence of childhood trauma. Whereas these SNPs did not modify depressive symptoms in the absence of ELS, they moderated the effect of childhood trauma, having either protective or harmful effects. Other studies underscore this influence of Crfr1 genotype on vulnerability for MDD, by showing either main effects of several yet different Crfr1 SNPs on risk for depressive symptoms and suicidality (Liu et al., 2006; Wasserman et al., 2009) or interactions with life stress (Liu et al., 2013; Davidow et al., 2014). Moreover, the Crfr1 genotype was found to predict antidepressant treatment response in an anxiety-dependent manner; treatment was most effective in highly anxious individuals with a specific Crfr1 haplotype (Licinio et al., 2004; Liu et al., 2007). A study in outbred mice performed to better understand these G x E interactions revealed a significant interaction between chronic stress and a Crfr1 SNP on basal HPA axis function (Labermaier et al., 2014). The risk haplotype carriers displayed an augmented increase in basal corticosterone levels as a consequence of chronic stress, whereas no differences in basal corticosterone levels were observed without a history of stress. Stress-naïve risk allele carriers showed increased Crfr1 mRNA expression in the pituitary, dentate gyrus, CA3 and all cortical layers, and increased CRFR1 binding in the pituitary, but not in the hippocampus or in the cortex. Treatment with a CRFR1 antagonist during the last 3 weeks of the 7-week chronic stress prevented the increase in HPA axis activity in the risk allele carriers. These data suggest that an individual’s Crfr1 genotype heavily determines not only one's vulnerability to stress but also one's sensitivity to treatment selectively antagonizing CRFR hyperactivity. Besides the Crfr1 gene, other reports suggest associations between genetic variation in the Crf and Crfr2 genes, although the evidence there is less compelling (for a review of these findings, see Binder and Nemeroff, 2010). In-depth characterization of all relevant variants will likely be important for improving our understanding of the interindividual differences in the long-term consequences of adverse experiences.

Epigenetic regulation of corticotropin-releasing factor system expression Epigenetic modifications in the central nervous system have been identified as one of the main mechanisms by which environmental stimuli such as stress can induce long-lasting alterations in neurobiological systems by influencing gene expression (Provencal and Binder, 2015), including the neuroendocrine system (Auger and Auger, 2013). The term “epigenetics” refers to all reversible chemical modifications of the chromatin structure that alter gene transcription without altering the DNA sequence, including DNA methylation, DNA hydroxymethylation, and histone modifications. Alterations in epigenetic regulation resulting from ELS have been suggested to contribute to the increased risk on stress-related mental disease by changing gene expression and thereby brain maturation during sensitive developmental stages (Murgatroyd et al., 2009). Stress in adulthood is also capable of inducing such changes (Dirven et al., 2017), which seem to reflect stress vulnerability. For example, a recent study by Sipahi et al. (2014) showed that DNMT1 gene methylation was increased in PTSD patients, relative to trauma-exposed controls (relating it to pathology), whereas pretrauma

Corticotropin-releasing factor/urocortin system mechanisms influencing resilience

241

methylation of a single DNMT3b CpG site predicted the later development of a traumainduced PTSD phenotype, indicative of preexisting vulnerability (Sipahi et al., 2014). Of particular interest for this chapter are the epigenetic mechanisms mediating longlasting alterations in the CRF/UCN system as a consequence of chronic stress. Previous work has indicated that chronic social stress long-lastingly decreased methylation of the Crf gene in the PVN, which was associated with increased basal Crf mRNA expression (Elliott et al., 2010). This demethylation was, however, only observed in animals that displayed social avoidance following defeat, and not in those resilient, implicating it as a mechanism for stress vulnerability. Immediately after defeat, decreased Dnmt3b (DNA methyltransferase 3b) and Hdac2 (histone deacetylase 2) expression and increased demethylation-promoting factor Gadd45 expression were found, with the latter potentially responsible for the demethylation. Antidepressant treatment attenuated social avoidance, as well as Crf expression and demethylation of its promoter. Thus, differential Crf gene demethylation reflects a mechanism mediating the behavioral consequences of stress, which explains other reports of increased PVN Crf mRNA expression in stress-susceptible mice only (Han et al., 2017) and is in line with the general demethylation of the Crf promoter region as a consequence of chronic variable mild stress (Sterrenburg et al., 2011). Similarly, the expression of Crfr1 is under epigenetic control. Chronic unpredictable stress was shown to reduce hypothalamic H3K9 trimethylation, which was associated with increased levels of local CRFR1 and avoidance behavior (Wan et al., 2014). In the amygdala, Crfr1 methylation suppresses Crfr1 mRNA expression by preventing the binding of the transcription factor Yin Yang 1 (YY1). Moreover, differences in methylation and thereby expression of Crfr1 in the amygdala were associated with both the distinct innate anxiety levels between animals and the anxiolytic and anxiogenic effects of environmental enrichment and chronic variable stress, respectively. Further evidence for the important role of epigenetic regulation of Crf expression is derived from studies on the stress resilienceepromoting effects of augmented maternal care. Augmented care reduced hypothalamic Crf expression by enhanced expression of the transcriptional repressor neuron restrictive silencing factor (NRSF) and its recruitment to the Crf gene (Korosi et al., 2010). This increased occupancy of NRSF at the Crf gene was joined by the recruitment of methyl CpG-binding protein 2 (MeCP2) binding, which typically binds methylated DNA and contributes to the repression of gene expression (McGill et al., 2006). Although this enriched NRSF and MeCP2 binding was relatively shortlasting, it induced an early and enduring increase in repressive epigenetic (i.e., methylation) marks and thereby long-lasting suppression of Crf expression (Singh-Taylor et al., 2018). These findings propose DNA methylation as a prominent mechanism by which either a beneficial or adverse environment can induce long-lasting neural and behavioral changes in stress sensitivity. MicroRNAs (miRs), which act as translational repressors, are considered another type of epigenetic modulator capable of influencing protein expression. Altered miR expression has in fact been proposed to mediate resilience to chronic stress (Issler and Chen, 2015). Inactivating miR processing by ablation of the Dicer gene in the CeA of adult mice was found to induce a robust increase in anxiety-like behavior (Haramati et al., 2011). As a follow-up, miR expression profiles were analyzed in response to stress, revealing several affected miRs with putative gene targets known to be associated with stress. One of the prominent stress-induced miRs found in this screen, miR-34c, was found upregulated after acute and

242

16. The role of the CRF-urocortin system in stress resilience

chronic stress and appeared to contribute to a reduction in anxiolytic behavior by targeting Crfr1 and thereby reducing neuronal responsiveness to CRF in neuronal cells endogenously expressing Crfr1. Considering the accumulating evidence for aberrant miR levels in patients suffering from stress-related psychopathology, such as PTSD (Zhou et al., 2014) and MDD (Lopez et al., 2014), as well as differences in other epigenetic regulators (Bagot et al., 2014), future research should further study the exact mechanisms by which these potent modulators encode stress vulnerability.

Stress regulation of CRFR1 availability One mechanism by which the effects of excessive or prolonged exposure to stress/CRF can be minimized is by the suppression of CRFR-induced signaling. Receptor phosphorylation, inducing its internalization and thereby rendering the cell relatively insensitive to CRF, is one of the main mechanisms by which this is established. Ligand binding triggers G proteinecoupled receptor kinases (GRKs) to rapidly phosphorylate the receptors, which desensitizes them and increases their affinity for b-arrestins by w30-fold. This ultimately triggers CRFR translocation to the cell surface, where b-arrestins uncouple the CRFR from the G protein and thereby “arrest” signal transduction. Moreover, b-arrestins enable the internalization of the desensitized CRFR1 (Gutknecht et al., 2009) and CRFR2 (Markovic et al., 2008), which are then either dephosphorylated in endosomes by specific phosphatases and recycled back to the plasma membrane, ordin case of prolonged exposure to high agonist concentrationsddegraded in lysosomes, resulting in a decrease in the total number of CRFRs (Kohout and Lefkowitz, 2003; Moore et al., 2007; Kelly et al., 2008). Importantly, not all phosphorylated receptors are internalized; some remain at the membrane to ensure proper response to recurrent exposure to stress (Krasel et al., 2005). Besides phosphorylation, actual CRFR bindingeinduced signaling is also modulated by interaction of a C-terminal PDZ-binding motif that is found in CRFR1, but not CRFR2. The binding of this motif to PDZ domains of membrane-associated guanylate kinases (MAGUKs) (among which postsynaptic density protein 95 [PSD95], synapse-associated protein 97 [SAP97], SAP102, PDZ domain containing 1 [PDZK1], and membrane-associated guanylate kinase, WW and PDZ domain containing 2 [MAGI2]) influences receptor localization in the cell by anchoring CRFR1 to larger signaling complexes (Bender et al., 2015; Walther et al., 2015) and affects receptor endocytosis (Dunn et al., 2016). Moreover, binding to the cystic fibrosis transmembrane conductance regulator-associated ligand (CAL) prevents CRFR1 trafficking to the cell surface and reduces its internalization via the modulation of the posttranslational modifications that the receptor undergoes within the Golgi apparatus (Hammad et al., 2015). Moreover, PDZ domainebased interactions seem to modulate downstream kinase phosphorylation (Walther et al., 2015; Hammad et al., 2015), as well as functional cross-talk between distinct receptors (Magalhaes et al., 2012). Besides these two main regulatory mechanisms, many other regulatory systems of CRFRs seem to modulate CRFR activity but are less well understood and deserve attention in the future, particularly with respect to modulation by prolonged/excessive stress exposure and vulnerability to stress-related psychopathology. That is, initial evidence implicates stress-induced alterations in these processes in the behavioral consequences of chronic stress exposure. Prolonged stress was found to desensitize CRFR1 and promote the

Corticotropin-releasing factor/urocortin system mechanisms influencing resilience

243

degradation of CRFR1s in the LC (Reyes et al., 2008). This region is activated by CRF and is hyperresponsive in MDD (Gold and Chrousos, 2002), which is characterized by increased LC CRF levels (Austin et al., 2003; Bissette et al., 2003; Merali et al., 2006). Local CRFR1 was internalized in response to CRF administration (Reyes et al., 2006), repeated stress exposure (Reyes et al., 2008), or chronic ethanol intake (Chaijale et al., 2013), which in turn attenuated the magnitude of the stress response by decreasing the postsynaptic sensitivity to CRF. This LC CRFR1 downregulation reflected an adaptive stress coping response to stress, as rats that displayed resilience to the learned helplessness model of depression were characterized by reduced CRFR1 levels in the LC and amygdala. Reduced receptor expression was associated with the maintenance of sufficient GRK3 levels, which were suppressed in helpless animals (Taneja et al., 2011). These suppressed levels of GRK3 could potentially be caused by increased sensitivity to oxidative stress in the helpless animals; protein carbonylation was increased in this subgroup specifically, and increased intracellular calcium as can be expected during excessive activation of neurons and oxidative stress is known to contribute to GRK3 degradation (Salim and Eikenburg, 2007). Notably, the reported internalization of LC CRFR1 only seems to occur in male rats, but not in female rats, in which the stress-induced CRFR association with b-arrestin 2, an integral step in receptor internalization, is not observed (Bangasser et al., 2010). This results in increased LC activity and suggests an impaired capacity for adaptation to stressors. However, the behavioral effect of CRF-induced CRFR1 desensitization is highly brain region specific. For example, in the BLA, chronic stressinduced a reduction in electrophysiological responsiveness to CRF, which was associated with increased anxiety- and depressive-like behavior (Sandi et al., 2008), whereas in the CeA, reduced expression of CRFR1 and thus reduced sensitivity to CRF was related to anxiolytic effects following stress exposure, potentially reflecting stress coping (Haramati et al., 2011).

Stress-induced changes in CRFR2 expression Differential levels of CRFR2 expression may also relate to interindividual differences in stress resilience. In a rat model for PTSD, the exposure to predator-associated cues (a psychological trauma) simulated several prevalent PTSD symptoms, including reexperiencing, avoidance, and hyperarousal, in a subset of susceptible animals, whereas others were relatively resilient (Elharrar et al., 2013). Interestingly, susceptible (PTSD-like) rats demonstrated not only an inability to suppress Crfr1 mRNA levels in the BNST but also a marked, long-term decrease in Crfr2 mRNA levels. Local upregulation of CRFR2 attenuated PTSD-like symptoms in the susceptible animals. In another study, (optogenetic) activation of CRFR2-expressing neurons in the posterior BNST reduced basal anxiety and contributed to stress and trauma recovery, reducing the risk on PTSD-like symptomatology afterward (Henckens et al., 2017). These findings seem in contrast to the observation of increased BNST Crfr2 mRNA expression associated with a PTSD susceptibility, which was rescued by BNST CRFR2-specific KD (Lebow et al., 2012). Potentially, these apparent inconsistencies can be explained by contrasting roles of different subnuclei and cell types within the BNST (Hammack et al., 2007) but nevertheless suggest a clear link between CRFR2-induced signaling and stress resilience. Other evidence for a role of interindividual differences in CRFR2 availability mediating stress sensitivity comes from studies in macaque monkeys,

244

16. The role of the CRF-urocortin system in stress resilience

where stress-sensitive animals tended to display lower CRFR2 mRNA expression in the dorsal raphe nucleus than highly stress-resilient ones. These levels were significantly unregulated upon selective serotonin reuptake inhibitor treatment in the stress-sensitive animals only (Bethea et al., 2011). When and how these differential expression patterns are generated and what their exact mechanistic base is remains unclear and deserves further investigation.

Corticotropin-releasing proteinebinding protein function The CRF-binding protein (CRF-BP) binds CRF with an affinity equal to or even higher than that of its receptors and therefore is an important indirect regulator of CRFR activation. In mice, CRF-BP expression is mainly found in the pituitary, cortex, hippocampus, amygdala, and BNST (Seasholtz et al., 2001). It is upregulated in response to stress in both the pituitary (McClennen et al., 1998) and amygdala (Lombardo et al., 2001; Herringa et al., 2004; Roseboom et al., 2007) and is thought to buffer the action of CRF in response to a current or subsequent stressor by sequestering the peptide, thereby preventing its interaction with the receptor and possibly targeting CRF for degradation (Behan et al., 1995). Pituitary CRF-BP OE mice showed a rather anxiolytic behavioral profile, whereas CRFBP-deficient mice displayed increased anxiogenic-like behavior in the elevated plus maze and defensive withdrawal tests (Burrows et al., 1998; Karolyi et al., 1999). However, these transgenic mouse lines are characterized by compensatory alterations in the CRF/UCN system (Burrows et al., 1998), limiting their value. Conversely, there is some indication that CRF-BPdwhen bound to CRFdmay act as a cellular signaling molecule, opening up the possibility that increased CRF-BP expression following stress exposure could in fact underlie the sensitization that occurs to some of the effects of stress (Ungless et al., 2003; Slater et al., 2016; Li et al., 2016). Several studies have implicated genetic variance (SNPs) in the CRF-BP gene to altered vulnerability to stress-related disorders (Van Den Eede, 2005). A specific CRF-BP genotype was associated with the risk for MDD (Claes et al., 2003), suicidal behavior (De Luca et al., 2010), anxiety and alcohol use disorders (Enoch et al., 2008), MDD and PTSD symptoms following treatment in the intensive care unit (Davydow et al., 2014), and cortisol stress responses in children (Sheikh et al., 2013). Moreover, the treatment response to antidepressants was modulated by genetic variance in the CRF-BP gene, an effect most pronounced in patients with anxious depression (Binder et al., 2010). Amygdala CRF-BP levels were increased in patients suffering from bipolar disorder and schizophrenia, but not MDD (Herringa et al., 2006). In contrast to this upregulation, animal work investigating differences between stressprone and stress-resilient genetic rat strains has found an overall downregulation of CRF-BP brain expression associated with increased stress sensitivity (Sabariego et al., 2011). Future studies investigating the link between CRF-BP availability and vulnerability to stress-related disease are therefore needed.

Alterations in intracellularly activated signaling pathways Activation of CRFRs can induce several distinct signaling pathways depending on their localization and cellular context. CRFRs primarily signal by G protein coupling, resulting in

Conclusion

245

the induction of cyclic AMP-protein kinase A (PKA) and the extracellular signaleregulated kinase-mitogen-activated protein kinase (ERK-MAPK) pathways. These signals induce intracellular calcium mobilization (Gutknecht et al., 2009), and the transcription of downstream target genes (Hauger et al., 2006), and thereby regulate synaptic plasticity processes such as dendrite stabilization, ion channel transmission, transcription of CREB and other genes, and receptor scaffolding, trafficking, and cross-talk. However, CRFRs also interact with other G protein systems, including Gq, Gi, Go, Gil/2, and Gz, by which they can activate phospholipase C (PLC) ultimately also resulting in the activation of ERK1 and ERK2 and an increase in intracellular calcium (Grammatopoulos et al., 2002). Chronic stress and drug exposure are capable of altering these downstream signaling pathways of CRFRs and modifying the effect of ligand-induced activation. This reflects another important mechanism by which chronic stress can long-lastingly alter the CRF/UCN system. For example, in the ventral tegmental area (VTA), CRFR1 activation typically initiates the PLC-protein kinase C (PKC) pathway (Wanat et al., 2008) but acts through the PKA pathway in drug-experienced animals (Hahn et al., 2009). Similar mechanisms may contribute to the observation that the effects of CRFR2 activation in the VTA are dependent on the prior history of an animal. Activation of presynaptic CRFR2 in naïve animals was found to facilitate presynaptic release of GABA and thereby suppressed VTA excitatory postsynaptic currents (EPSCs). However, after chronic cocaine self-administration and extinction training, the ability of CRFR2 agonists to depress EPSCs and potentiate inhibitory postsynaptic currents was diminished. Administration of yohimbine (an a-receptor antagonist, increasing circulating levels of noradrenaline) and cue reinstatement reversed the effects of CRFR2 on GABA and glutamate release; EPSCs were increased as a result of a reduction of tonic GABA-dependent inhibition (Williams et al., 2014). Similarly, in the lateral septum, CRFR2 activation induces a PKA-dominant pathway, which is changed into a PKC-dominant pathway following chronic cocaine administration and withdrawal. This changes the functional consequences of receptor activation; the CRFR2-mediated depression of excitatory glutamatergic transmission induced by UCN1 was switched to a facilitation with a comparable potency (Liu et al., 2005; Gallagher et al., 2008). Such alterations in CRF/UCN-induced cellular signaling may be integral for developing resilience to stressinduced depression. Social defeat-susceptible rats developed adaptations in their HPA axis response more slowly following chronic stress than resilient animals, due to loss of response to CRF in the presence of normal CRFR concentrations, suggestive of receptor uncoupling or attenuation of cellular signaling (Wood et al., 2010).

Conclusion Although an organism may benefit from the initial CRF/UCN system response in the presence of acute threat, prolonged stress exposure or stress experienced during critical periods in development lastingly alter the system, thereby contributing to an autonomic, neuroendocrine, and behavioral state very much resembling that of stress-related psychopathology. Abundant human evidence substantiates this presumed CRF/UCN system dysfunction in stress-related mental disorders such as MDD and PTSD. Mechanisms mediating the transition to disease seem to be subject to substantial interindividual variation

246

16. The role of the CRF-urocortin system in stress resilience

FIGURE 16.2 CRF/UCN system mechanisms influencing stress resilience versus susceptibility. Evidence suggests that a stressful environment increases risk for disease in susceptible individuals by interacting with one’s genetic background of the CRF/UCN system (e.g., in CRFR1) (1), as well as by inducing epigenetic changes regulating CRFR and ligand expression (2). Moreover, interindividual differences in stress-mediated regulation of CRFR1 (modulated by receptor phosphorylation and PDZ domain interactions (3)) and CRFR2 availability (4), as well as the regulation of available ligand by CRF-BP (5), contribute to risk on disease. Lastly, stress exposure can (lastingly) alter CRF/UCN-mediated activation of distinct signaling pathways (6), which might additionally contribute to stress-related mental disorders. A, acetylation; cAMP, cyclic AMP; DAG, diacylglycerol; ERK, extracellular signaleregulated kinase; GRK, G proteinecoupled receptor kinase; IP3, inositol-1,4,5-triphosphate; M, methylation; MAGUK, membrane-associated guanylate kinase; MAPK, mitogen-activated protein kinase; PKA, protein kinase A; PLC, phospholipase C.

and determined by several factors (Fig. 16.2): first of all, the genetic background of the individual, which interacts with the environment in mediating the risk for disease; secondly, by stress-induced epigenetic changes regulating CRFR and ligand expression that may vary among individuals; thirdly, by differential regulation of CRFR1 (modulated by receptor phosphorylation and PDZ domain interactions) and CRFR2 availability and the regulation of available ligands by CRF-BP; and lastly, by stress-induced alterations in activated signaling pathways, which may mediate the interindividual differences in susceptibility to stressrelated disease. Increased understanding of these exact mechanisms would aid identification of at-risk individuals and improve treatment options for those suffering from stress-related psychiatric disorders.

References Afifi, T.O., Asmundson, G.J., Taylor, S., Jang, K.L., 2010. The role of genes and environment on trauma exposure and posttraumatic stress disorder symptoms: a review of twin studies. Clinical Psychology Review 30 (1), 101e112. Andres, A.L., Regev, L., Phi, L., Seese, R.R., Chen, Y., Gall, C.M., Baram, T.Z., 2013. NMDA receptor activation and calpain contribute to disruption of dendritic spines by the stress neuropeptide CRH. Journal of Neuroscience 33 (43), 16945e16960. Anisman, H., Prakash, P., Merali, Z., Poulter, M.O., 2007. Corticotropin releasing hormone receptor alterations elicited by acute and chronic unpredictable stressor challenges in stressor-susceptible and resilient strains of mice. Behavioural Brain Research 181 (2), 180e190.

References

247

Auger, C.J., Auger, A.P., 2013. Permanent and plastic epigenesis in neuroendocrine systems. Frontiers in Neuroendocrinology 34 (3), 190e197. Austin, M.C., Janosky, J.E., Murphy, H.A., 2003. Increased corticotropin-releasing hormone immunoreactivity in monoamine-containing pontine nuclei of depressed suicide men. Molecular Psychiatry 8 (3), 324e332. Bagot, R.C., Labonte, B., Pena, C.J., Nestler, E.J., 2014. Epigenetic signaling in psychiatric disorders: stress and depression. Dialogues in Clinical Neuroscience 16 (3), 281e295. Baker, A., Shalhoub-Kevorkian, N., 1999. Effects of political and military traumas on children: the Palestinian case. Clinical Psychology Review 19 (8), 935e950. Bale, T.L., Contarino, A., Smith, G.W., Chan, R., Gold, L.H., Sawchenko, P.E., et al., 2000. Mice deficient for corticotropin-releasing hormone receptor-2 display anxiety-like behaviour and are hypersensitive to stress. Nature Genetics 24 (4), 410e414. Bale, T.L., Vale, W.W., 2004. CRF and CRF receptors: role in stress responsivity and other behaviors. Annual Review of Pharmacology and Toxicology 44, 525e557. Bangasser, D.A., Curtis, A., Reyes, B.A., Bethea, T.T., Parastatidis, I., Ischiropoulos, H., et al., 2010. Sex differences in corticotropin-releasing factor receptor signaling and trafficking: potential role in female vulnerability to stressrelated psychopathology. Molecular Psychiatry 15 (9), 877, 896e904. Banki, C.M., Karmacsi, L., Bissette, G., Nemeroff, C.B., 1992. CSF corticotropin-releasing hormone and somatostatin in major depression: response to antidepressant treatment and relapse. European Neuropsychopharmacology 2 (2), 107e113. Behan, D.P., De Souza, E.B., Lowry, P.J., Potter, E., Sawchenko, P., Vale, W.W., 1995. Corticotropin releasing factor (CRF) binding protein: a novel regulator of CRF and related peptides. Frontiers in Neuroendocrinology 16 (4), 362e382. Bender, J., Engeholm, M., Ederer, M.S., Breu, J., Moller, T.C., Michalakis, S., et al., 2015. Corticotropin-releasing hormone receptor type 1 (CRHR1) clustering with MAGUKs is mediated via its C-terminal PDZ binding motif. PLoS One 10 (9), e0136768. Bethea, C.L., Lima, F.B., Centeno, M.L., Weissheimer, K.V., Senashova, O., Reddy, A.P., Cameron, J.L., 2011. Effects of citalopram on serotonin and CRF systems in the midbrain of primates with differences in stress sensitivity. Journal of Chemical Neuroanatomy 41 (4), 200e218. Bijlsma, E.Y., van Leeuwen, M.L., Westphal, K.G., Olivier, B., Groenink, L., 2011. Local repeated corticotropinreleasing factor infusion exacerbates anxiety- and fear-related behavior: differential involvement of the basolateral amygdala and medial prefrontal cortex. Neuroscience 173, 82e92. Binder, E.B., Nemeroff, C.B., 2010. The CRF system, stress, depression and anxiety-insights from human genetic studies. Molecular Psychiatry 15 (6), 574e588. https://doi.org/10.1038/mp.2009.141. Binder, E.B., Owens, M.J., Liu, W., Deveau, T.C., Rush, A.J., Trivedi, M.H., et al., 2010. Association of polymorphisms in genes regulating the corticotropin-releasing factor system with antidepressant treatment response. Archives of General Psychiatry 67 (4), 369e379. Bingham, B.C., Sheela Rani, C.S., Frazer, A., Strong, R., Morilak, D.A., 2013. Exogenous prenatal corticosterone exposure mimics the effects of prenatal stress on adult brain stress response systems and fear extinction behavior. Psychoneuroendocrinology 38 (11), 2746e2757. Binneman, B., Feltner, D., Kolluri, S., Shi, Y., Qiu, R., Stiger, T., 2008. A 6-week randomized, placebo-controlled trial of CP-316,311 (a selective CRH1 antagonist) in the treatment of major depression. American Journal of Psychiatry 165 (5), 617e620. https://doi.org/10.1176/appi.ajp.2008.07071199. Bissette, G., Klimek, V., Pan, J., Stockmeier, C., Ordway, G., 2003. Elevated concentrations of CRF in the locus coeruleus of depressed subjects. Neuropsychopharmacology 28 (7), 1328e1335. Blank, T., Nijholt, I., Eckart, K., Spiess, J., 2002. Priming of long-term potentiation in mouse hippocampus by corticotropin-releasing factor and acute stress: implications for hippocampus-dependent learning. Journal of Neuroscience 22 (9), 3788e3794. Bradley, R.G., Binder, E.B., Epstein, M.P., Tang, Y., Nair, H.P., Liu, W., et al., 2008. Influence of child abuse on adult depression: moderation by the corticotropin-releasing hormone receptor gene. Archives of General Psychiatry 65 (2), 190e200. Bremner, J.D., Licinio, J., Darnell, A., Krystal, J.H., Owens, M.J., Southwick, S.M., et al., 1997. Elevated CSF corticotropin-releasing factor concentrations in posttraumatic stress disorder. American Journal of Psychiatry 154 (5), 624e629.

248

16. The role of the CRF-urocortin system in stress resilience

Brewin, C.R., Kleiner, J.S., Vasterling, J.J., Field, A.P., 2007. Memory for emotionally neutral information in posttraumatic stress disorder: a meta-analytic investigation. Journal of Abnormal Psychology 116 (3), 448e463. Brown, M.R., Fisher, L.A., Spiess, J., Rivier, C., Rivier, J., Vale, W., 1982. Corticotropin-releasing factor: actions on the sympathetic nervous system and metabolism. Endocrinology 111 (3), 928e931. Burrows, H.L., Nakajima, M., Lesh, J.S., Goosens, K.A., Samuelson, L.C., Inui, A., et al., 1998. Excess corticotropin releasing hormone-binding protein in the hypothalamic-pituitary-adrenal axis in transgenic mice. Journal of Clinical Investigation 101 (7), 1439e1447. Chaijale, N.N., Curtis, A.L., Wood, S.K., Zhang, X.Y., Bhatnagar, S., Reyes, B.A., et al., 2013. Social stress engages opioid regulation of locus coeruleus norepinephrine neurons and induces a state of cellular and physical opiate dependence. Neuropsychopharmacology 38 (10), 1833e1843. Chappell, P.B., Smith, M.A., Kilts, C.D., Bissette, G., Ritchie, J., Anderson, C., Nemeroff, C.B., 1986. Alterations in corticotropin-releasing factor-like immunoreactivity in discrete rat brain regions after acute and chronic stress. Journal of Neuroscience 6 (10), 2908e2914. Chen, Y., Brunson, K.L., Adelmann, G., Bender, R.A., Frotscher, M., Baram, T.Z., 2004. Hippocampal corticotropin releasing hormone: pre- and postsynaptic location and release by stress. Neuroscience 126 (3), 533e540. Chen, Y., Kramar, E.A., Chen, L.Y., Babayan, A.H., Andres, A.L., Gall, C.M., et al., 2013. Impairment of synaptic plasticity by the stress mediator CRH involves selective destruction of thin dendritic spines via RhoA signaling. Molecular Psychiatry 18 (4), 485e496. Claes, S., Villafuerte, S., Forsgren, T., Sluijs, S., Del-Favero, J., Adolfsson, R., Van Broeckhoven, C., 2003. The corticotropin-releasing hormone binding protein is associated with major depression in a population from Northern Sweden. Biological Psychiatry 54 (9), 867e872. Contarino, A., Dellu, F., Koob, G.F., Smith, G.W., Lee, K.F., Vale, W., Gold, L.H., 1999. Reduced anxiety-like and cognitive performance in mice lacking the corticotropin-releasing factor receptor 1. Brain Research 835 (1), 1e9. Coste, S.C., Kesterson, R.A., Heldwein, K.A., Stevens, S.L., Heard, A.D., Hollis, J.H., et al., 2000. Abnormal adaptations to stress and impaired cardiovascular function in mice lacking corticotropin-releasing hormone receptor-2. Nature Genetics 24 (4), 403e409. Dautzenberg, F.M., Hauger, R.L., 2002. The CRF peptide family and their receptors: yet more partners discovered. Trends in Pharmacological Sciences 23 (2), 71e77. Davis, M., 1998. Are different parts of the extended amygdala involved in fear versus anxiety? Biological Psychiatry 44 (12), 1239e1247. Davydow, D.S., Kohen, R., Hough, C.L., Tracy, J.H., Zatzick, D., Katon, W.J., 2014. A pilot investigation of the association of genetic polymorphisms regulating corticotrophin-releasing hormone with posttraumatic stress and depressive symptoms in medical-surgical intensive care unit survivors. Journal of Critical Care 29 (1), 101e106. de Andrade, J.S., Cespedes, I.C., Abrao, R.O., Dos Santos, T.B., Diniz, L., Britto, L.R., et al., 2013. Chronic unpredictable mild stress alters an anxiety-related defensive response, Fos immunoreactivity and hippocampal adult neurogenesis. Behavioural Brain Research 250, 81e90. De Bellis, M.D., Gold, P.W., Geracioti Jr., T.D., Listwak, S.J., Kling, M.A., 1993. Association of fluoxetine treatment with reductions in CSF concentrations of corticotropin-releasing hormone and arginine vasopressin in patients with major depression. American Journal of Psychiatry 150 (4), 656e657. de Kloet, E.R., Joels, M., Holsboer, F., 2005. Stress and the brain: from adaptation to disease. Nature Reviews Neuroscience 6 (6), 463e475. De Luca, V., Tharmalingam, S., Zai, C., Potapova, N., Strauss, J., Vincent, J., Kennedy, J.L., 2010. Association of HPA axis genes with suicidal behaviour in schizophrenia. Journal of Psychopharmacology 24 (5), 677e682. Dirven, B.C.J., Homberg, J.R., Kozicz, T., Henckens, M., 2017. Epigenetic programming of the neuroendocrine stress response by adult life stress. Journal of Molecular Endocrinology 59 (1), R11eR31. Dunlop, B.W., Binder, E.B., Iosifescu, D., Mathew, S.J., Neylan, T.C., Pape, J.C., et al., 2017. Corticotropin-releasing factor receptor 1 antagonism is ineffective for women with posttraumatic stress disorder. Biological Psychiatry 82 (12), 866e874. Dunn, A.J., Berridge, C.W., 1990. Is corticotropin-releasing factor a mediator of stress responses? Annals of the New York Academy of Sciences 579, 183e191. Dunn, H.A., Chahal, H.S., Caetano, F.A., Holmes, K.D., Yuan, G.Y., Parikh, R., et al., 2016. PSD-95 regulates CRFR1 localization, trafficking and beta-arrestin2 recruitment. Cellular Signalling 28 (5), 531e540.

References

249

Elharrar, E., Warhaftig, G., Issler, O., Sztainberg, Y., Dikshtein, Y., Zahut, R., et al., 2013. Overexpression of corticotropin-releasing factor receptor type 2 in the bed nucleus of stria terminalis improves posttraumatic stress disorder-like symptoms in a model of incubation of fear. Biological Psychiatry 74 (11), 827e836. Elliott, E., Ezra-Nevo, G., Regev, L., Neufeld-Cohen, A., Chen, A., 2010. Resilience to social stress coincides with functional DNA methylation of the Crf gene in adult mice. Nature Neuroscience 13 (11), 1351e1353. Enoch, M.A., Shen, P.H., Ducci, F., Yuan, Q., Liu, J., White, K.V., et al., 2008. Common genetic origins for EEG, alcoholism and anxiety: the role of CRH-BP. PLoS One 3 (10), e3620. Fan, J.M., Wang, X., Hao, K., Yuan, Y., Chen, X.Q., Du, J.Z., 2013. Upregulation of PVN CRHR1 by gestational intermittent hypoxia selectively triggers a male-specific anxiogenic effect in rat offspring. Hormones and Behavior 63 (1), 25e31. Gallagher, J.P., Orozco-Cabal, L.F., Liu, J., Shinnick-Gallagher, P., 2008. Synaptic physiology of central CRH system. European Journal of Pharmacology 583 (2e3), 215e225. Gehlert, D.R., Shekhar, A., Morin, S.M., Hipskind, P.A., Zink, C., Gackenheimer, S.L., et al., 2005. Stress and central Urocortin increase anxiety-like behavior in the social interaction test via the CRF1 receptor. European Journal of Pharmacology 509 (2e3), 145e153. Glover, E.M., Phifer, J.E., Crain, D.F., Norrholm, S.D., Davis, M., Bradley, B., et al., 2011. Tools for translational neuroscience: PTSD is associated with heightened fear responses using acoustic startle but not skin conductance measures. Depression and Anxiety 28 (12), 1058e1066. Gold, P.W., Chrousos, G.P., 2002. Organization of the stress system and its dysregulation in melancholic and atypical depression: high vs low CRH/NE states. Molecular Psychiatry 7 (3), 254e275. Graham, D.L., Grace, C.E., Braun, A.A., Schaefer, T.L., Skelton, M.R., Tang, P.H., et al., 2011. Effects of developmental stress and lead (Pb) on corticosterone after chronic and acute stress, brain monoamines, and blood Pb levels in rats. International Journal of Developmental Neuroscience 29 (1), 45e55. Grammatopoulos, D.K., Chrousos, G.P., 2002. Functional characteristics of CRH receptors and potential clinical applications of CRH-receptor antagonists. Trends in Endocrinology and Metabolism 13 (10), 436e444. Gunn, B.G., Cunningham, L., Cooper, M.A., Corteen, N.L., Seifi, M., Swinny, J.D., et al., 2013. Dysfunctional astrocytic and synaptic regulation of hypothalamic glutamatergic transmission in a mouse model of early-life adversity: relevance to neurosteroids and programming of the stress response. Journal of Neuroscience 33 (50), 19534e19554. Gutknecht, E., Van der Linden, I., Van Kolen, K., Verhoeven, K.F., Vauquelin, G., Dautzenberg, F.M., 2009. Molecular mechanisms of corticotropin-releasing factor receptor-induced calcium signaling. Molecular Pharmacology 75 (3), 648e657. Habib, K.E., Weld, K.P., Rice, K.C., Pushkas, J., Champoux, M., Listwak, S., et al., 2000. Oral administration of a corticotropin-releasing hormone receptor antagonist significantly attenuates behavioral, neuroendocrine, and autonomic responses to stress in primates. Proceedings of the National Academy of Sciences of the United States of America 97 (11), 6079e6084. Hahn, J., Hopf, F.W., Bonci, A., 2009. Chronic cocaine enhances corticotropin-releasing factor-dependent potentiation of excitatory transmission in ventral tegmental area dopamine neurons. Journal of Neuroscience 29 (20), 6535e6544. Hammack, S.E., Mania, I., Rainnie, D.G., 2007. Differential expression of intrinsic membrane currents in defined cell types of the anterolateral bed nucleus of the stria terminalis. Journal of Neurophysiology 98 (2), 638e656. Hammad, M.M., Dunn, H.A., Walther, C., Ferguson, S.S., 2015. Role of cystic fibrosis transmembrane conductance regulator-associated ligand (CAL) in regulating the trafficking and signaling of corticotropin-releasing factor receptor 1. Cellular Signalling 27 (10), 2120e2130. Han, Q.Q., Yang, L., Huang, H.J., Wang, Y.L., Yu, R., Wang, J., et al., 2017. Differential GR expression and translocation in the Hippocampus mediates susceptibility vs. Resilience to chronic social defeat stress. Frontiers in Neuroscience 11, 287. Haramati, S., Navon, I., Issler, O., Ezra-Nevo, G., Gil, S., Zwang, R., et al., 2011. MicroRNA as repressors of stress-induced anxiety: the case of amygdalar miR-34. Journal of Neuroscience 31 (40), 14191e14203. Hartline, K.M., Owens, M.J., Nemeroff, C.B., 1996. Postmortem and cerebrospinal fluid studies of corticotropinreleasing factor in humans. Annals of the New York Academy of Sciences 780, 96e105.

250

16. The role of the CRF-urocortin system in stress resilience

Hauger, R.L., Risbrough, V., Brauns, O., Dautzenberg, F.M., 2006. Corticotropin releasing factor (CRF) receptor signaling in the central nervous system: new molecular targets. CNS and Neurological Disorders e Drug Targets 5 (4), 453e479. Heinrichs, S.C., Koob, G.F., 2004. Corticotropin-releasing factor in brain: a role in activation, arousal, and affect regulation. Journal of Pharmacology and Experimental Therapeutics 311 (2), 427e440. Henckens, M., Printz, Y., Shamgar, U., Dine, J., Lebow, M., Drori, Y., et al., 2017. CRF receptor type 2 neurons in the posterior bed nucleus of the stria terminalis critically contribute to stress recovery. Molecular Psychiatry 22 (12), 1691e1700. Henckens, M.J., Deussing, J.M., Chen, A., 2016. Region-specific roles of the corticotropin-releasing factor-urocortin system in stress. Nature Reviews Neuroscience 17 (10), 636e651. Henckens, M.J., van der Marel, K., van der Toorn, A., Pillai, A.G., Fernandez, G., Dijkhuizen, R.M., Joels, M., 2015. Stress-induced alterations in large-scale functional networks of the rodent brain. NeuroImage 105, 312e322. Herman, J.P., Adams, D., Prewitt, C., 1995. Regulatory changes in neuroendocrine stress-integrative circuitry produced by a variable stress paradigm. Neuroendocrinology 61 (2), 180e190. Herman, J.P., Tasker, J.G., 2016. Paraventricular hypothalamic mechanisms of chronic stress adaptation. Frontiers in Endocrinology 7, 137. Herman, J.P., Wiegand, S.J., Watson, S.J., 1990. Regulation of basal corticotropin-releasing hormone and arginine vasopressin messenger ribonucleic acid expression in the paraventricular nucleus: effects of selective hypothalamic deafferentations. Endocrinology 127 (5), 2408e2417. Herringa, R.J., Nanda, S.A., Hsu, D.T., Roseboom, P.H., Kalin, N.H., 2004. The effects of acute stress on the regulation of central and basolateral amygdala CRF-binding protein gene expression. Molecular Brain Research 131 (1e2), 17e25. Herringa, R.J., Roseboom, P.H., Kalin, N.H., 2006. Decreased amygdala CRF-binding protein mRNA in post-mortem tissue from male but not female bipolar and schizophrenic subjects. Neuropsychopharmacology 31 (8), 1822e1831. Heuser, I., Bissette, G., Dettling, M., Schweiger, U., Gotthardt, U., Schmider, J., et al., 1998. Cerebrospinal fluid concentrations of corticotropin-releasing hormone, vasopressin, and somatostatin in depressed patients and healthy controls: response to amitriptyline treatment. Depression and Anxiety 8 (2), 71e79. Hoffman, A.N., Lorson, N.G., Sanabria, F., Foster Olive, M., Conrad, C.D., 2014. Chronic stress disrupts fear extinction and enhances amygdala and hippocampal Fos expression in an animal model of post-traumatic stress disorder. Neurobiology of Learning and Memory 112, 139e147. Holsboer, F., 1999. The rationale for corticotropin-releasing hormone receptor (CRH-R) antagonists to treat depression and anxiety. Journal of Psychiatric Research 33 (3), 181e214. Holsboer, F., 2003. Corticotropin-releasing hormone modulators and depression. Current Opinion in Investigational Drugs 4 (1), 46e50. Hubbard, D.T., Nakashima, B.R., Lee, I., Takahashi, L.K., 2007. Activation of basolateral amygdala corticotropinreleasing factor 1 receptors modulates the consolidation of contextual fear. Neuroscience 150 (4), 818e828. Hung, H.C., Chou, C.K., Chiu, T.H., Lee, E.H., 1992. CRF increases protein phosphorylation and enhances retention performance in rats. NeuroReport 3 (2), 181e184. Ising, M., Zimmermann, U.S., Kunzel, H.E., Uhr, M., Foster, A.C., Learned-Coughlin, S.M., et al., 2007. High-affinity CRF1 receptor antagonist NBI-34041: preclinical and clinical data suggest safety and efficacy in attenuating elevated stress response. Neuropsychopharmacology 32 (9), 1941e1949. Issler, O., Carter, R.N., Paul, E.D., Kelly, P.A., Olverman, H.J., Neufeld-Cohen, A., et al., 2014. Increased anxiety in corticotropin-releasing factor type 2 receptor-null mice requires recent acute stress exposure and is associated with dysregulated serotonergic activity in limbic brain areas. Biology of Mood and Anxiety Disorders 4 (1), 1. Issler, O., Chen, A., 2015. Determining the role of microRNAs in psychiatric disorders. Nature Reviews Neuroscience 16 (4), 201e212. Itoga, C.A., Roltsch Hellard, E.A., Whitaker, A.M., Lu, Y.L., Schreiber, A.L., Baynes, B.B., et al., 2016. Traumatic stress promotes hyperalgesia via corticotropin-releasing factor-1 receptor (CRFR1) signaling in central amygdala. Neuropsychopharmacology 41 (10), 2463e2472. Ivy, A.S., Rex, C.S., Chen, Y., Dube, C., Maras, P.M., Grigoriadis, D.E., et al., 2010. Hippocampal dysfunction and cognitive impairments provoked by chronic early-life stress involve excessive activation of CRH receptors. Journal of Neuroscience 30 (39), 13005e13015.

References

251

Ji, G., Fu, Y., Adwanikar, H., Neugebauer, V., 2013. Non-pain-related CRF1 activation in the amygdala facilitates synaptic transmission and pain responses. Molecular Pain 9, 2. Ji, G., Fu, Y., Ruppert, K.A., Neugebauer, V., 2007. Pain-related anxiety-like behavior requires CRF1 receptors in the amygdala. Molecular Pain 3, 13. Jochman, K.A., Newman, S.M., Kalin, N.H., Bakshi, V.P., 2005. Corticotropin-releasing factor-1 receptors in the basolateral amygdala mediate stress-induced anorexia. Behavioral Neuroscience 119 (6), 1448e1458. Karolyi, I.J., Burrows, H.L., Ramesh, T.M., Nakajima, M., Lesh, J.S., Seong, E., et al., 1999. Altered anxiety and weight gain in corticotropin-releasing hormone-binding protein-deficient mice. Proceedings of the National Academy of Sciences of the United States of America 96 (20), 11595e11600. Keck, M.E., 2006. Corticotropin-releasing factor, vasopressin and receptor systems in depression and anxiety. Amino Acids 31 (3), 241e250. Keck, M.E., Ohl, F., Holsboer, F., Muller, M.B., 2005. Listening to mutant mice: a spotlight on the role of CRF/CRF receptor systems in affective disorders. Neuroscience and Biobehavioral Reviews 29 (4e5), 867e889. Keen-Rhinehart, E., Michopoulos, V., Toufexis, D.J., Martin, E.I., Nair, H., Ressler, K.J., et al., 2009. Continuous expression of corticotropin-releasing factor in the central nucleus of the amygdala emulates the dysregulation of the stress and reproductive axes. Molecular Psychiatry 14 (1), 37e50. Kelly, E., Bailey, C.P., Henderson, G., 2008. Agonist-selective mechanisms of GPCR desensitization. British Journal of Pharmacology 153 (Suppl. 1), S379e388. Kishimoto, T., Radulovic, J., Radulovic, M., Lin, C.R., Schrick, C., Hooshmand, F., et al., 2000. Deletion of crhr2 reveals an anxiolytic role for corticotropin-releasing hormone receptor-2. Nature Genetics 24 (4), 415e419. Kohout, T.A., Lefkowitz, R.J., 2003. Regulation of G protein-coupled receptor kinases and arrestins during receptor desensitization. Molecular Pharmacology 63 (1), 9e18. Koob, G.F., Heinrichs, S.C., 1999. A role for corticotropin releasing factor and urocortin in behavioral responses to stressors. Brain Research 848 (1e2), 141e152. Korosi, A., Shanabrough, M., McClelland, S., Liu, Z.W., Borok, E., Gao, X.B., et al., 2010. Early-life experience reduces excitation to stress-responsive hypothalamic neurons and reprograms the expression of corticotropin-releasing hormone. Journal of Neuroscience 30 (2), 703e713. Krasel, C., Bunemann, M., Lorenz, K., Lohse, M.J., 2005. Beta-arrestin binding to the beta2-adrenergic receptor requires both receptor phosphorylation and receptor activation. Journal of Biological Chemistry 280 (10), 9528e9535. Krishnan, V., Han, M.H., Graham, D.L., Berton, O., Renthal, W., Russo, S.J., et al., 2007. Molecular adaptations underlying susceptibility and resistance to social defeat in brain reward regions. Cell 131 (2), 391e404. Labermaier, C., Kohl, C., Hartmann, J., Devigny, C., Altmann, A., Weber, P., et al., 2014. A polymorphism in the Crhr1 gene determines stress vulnerability in male mice. Endocrinology 155 (7), 2500e2510. Lancel, M., Muller-Preuss, P., Wigger, A., Landgraf, R., Holsboer, F., 2002. The CRH1 receptor antagonist R121919 attenuates stress-elicited sleep disturbances in rats, particularly in those with high innate anxiety. Journal of Psychiatric Research 36 (4), 197e208. Lebow, M., Neufeld-Cohen, A., Kuperman, Y., Tsoory, M., Gil, S., Chen, A., 2012. Susceptibility to PTSD-like behavior is mediated by corticotropin-releasing factor receptor type 2 levels in the bed nucleus of the stria terminalis. Journal of Neuroscience 32 (20), 6906e6916. LeDoux, J.E., Iwata, J., Cicchetti, P., Reis, D.J., 1988. Different projections of the central amygdaloid nucleus mediate autonomic and behavioral correlates of conditioned fear. Journal of Neuroscience 8 (7), 2517e2529. Li, C., Liu, Y., Liu, D., Jiang, H., Pan, F., 2016. Dynamic alterations of miR-34c expression in the hypothalamus of male rats after early adolescent traumatic stress. Neural Plasticity 2016, 5249893. Licinio, J., O’Kirwan, F., Irizarry, K., Merriman, B., Thakur, S., Jepson, R., Lake, S., Tantisira, K.G., Weiss, S.T., Wong, M.L., 2004. Association of a corticotropin-releasing hormone receptor 1 haplotype and antidepressant treatment response in Mexican-Americans. Molecular Psychiatry 9 (12), 1075e1082. Liebsch, G., Landgraf, R., Engelmann, M., Lorscher, P., Holsboer, F., 1999. Differential behavioural effects of chronic infusion of CRH 1 and CRH 2 receptor antisense oligonucleotides into the rat brain. Journal of Psychiatric Research 33 (2), 153e163. Liebsch, G., Landgraf, R., Gerstberger, R., Probst, J.C., Wotjak, C.T., Engelmann, M., et al., 1995. Chronic infusion of a CRH1 receptor antisense oligodeoxynucleotide into the central nucleus of the amygdala reduced anxiety-related behavior in socially defeated rats. Regulatory Peptides 59 (2), 229e239.

252

16. The role of the CRF-urocortin system in stress resilience

Liposits, Z., Uht, R.M., Harrison, R.W., Gibbs, F.P., Paull, W.K., Bohn, M.C., 1987. Ultrastructural localization of glucocorticoid receptor (GR) in hypothalamic paraventricular neurons synthesizing corticotropin releasing factor (CRF). Histochemistry 87 (5), 407e412. Liu, J., Yu, B., Orozco-Cabal, L., Grigoriadis, D.E., Rivier, J., Vale, W.W., et al., 2005. Chronic cocaine administration switches corticotropin-releasing factor2 receptor-mediated depression to facilitation of glutamatergic transmission in the lateral septum. Journal of Neuroscience 25 (3), 577e583. Liu, Z., Liu, W., Yao, L., Yang, C., Xiao, L., Wan, Q., et al., 2013. Negative life events and corticotropinreleasing-hormone receptor1 gene in recurrent major depressive disorder. Scientific Reports 3, 1548. https:// doi.org/10.1038/srep01548. Liu, Z., Zhu, F., Wang, G., Xiao, Z., Tang, J., Liu, W., et al., 2007. Association study of corticotropin-releasing hormone receptor1 gene polymorphisms and antidepressant response in major depressive disorders. Neuroscience Letters 414 (2), 155e158. Liu, Z., Zhu, F., Wang, G., Xiao, Z., Wang, H., Tang, J., et al., 2006. Association of corticotropin-releasing hormone receptor1 gene SNP and haplotype with major depression. Neuroscience Letters 404 (3), 358e362. Lombardo, K.A., Herringa, R.J., Balachandran, J.S., Hsu, D.T., Bakshi, V.P., Roseboom, P.H., Kalin, N.H., 2001. Effects of acute and repeated restraint stress on corticotropin-releasing hormone binding protein mRNA in rat amygdala and dorsal hippocampus. Neuroscience Letters 302 (2e3), 81e84. Lopez, J.P., Lim, R., Cruceanu, C., Crapper, L., Fasano, C., Labonte, B., et al., 2014. miR-1202 is a primate-specific and brain-enriched microRNA involved in major depression and antidepressant treatment. Nature Medicine 20 (7), 764e768. https://doi.org/10.1038/nm.3582. Magalhaes, A.C., Dunn, H., Ferguson, S.S., 2012. Regulation of GPCR activity, trafficking and localization by GPCR-interacting proteins. British Journal of Pharmacology 165 (6), 1717e1736. Makino, S., Smith, M.A., Gold, P.W., 1995. Increased expression of corticotropin-releasing hormone and vasopressin messenger ribonucleic acid (mRNA) in the hypothalamic paraventricular nucleus during repeated stress: association with reduction in glucocorticoid receptor mRNA levels. Endocrinology 136 (8), 3299e3309. Markovic, D., Punn, A., Lehnert, H., Grammatopoulos, D.K., 2008. Intracellular mechanisms regulating corticotropinreleasing hormone receptor-2beta endocytosis and interaction with extracellularly regulated kinase 1/2 and p38 mitogen-activated protein kinase signaling cascades. Molecular Endocrinology 22 (3), 689e706. McClennen, S.J., Cortright, D.N., Seasholtz, A.F., 1998. Regulation of pituitary corticotropin-releasing hormonebinding protein messenger ribonucleic acid levels by restraint stress and adrenalectomy. Endocrinology 139 (11), 4435e4441. McGill, B.E., Bundle, S.F., Yaylaoglu, M.B., Carson, J.P., Thaller, C., Zoghbi, H.Y., 2006. Enhanced anxiety and stressinduced corticosterone release are associated with increased Crh expression in a mouse model of Rett syndrome. Proceedings of the National Academy of Sciences of the United States of America 103 (48), 18267e18272. McEwen, B.S., 2012. Brain on stress: how the social environment gets under the skin. Proceedings of the National Academy of Sciences of the United States of America 109 (Suppl. 2), 17180e17185. McLaughlin, J.P., Li, S., Valdez, J., Chavkin, T.A., Chavkin, C., 2006. Social defeat stress-induced behavioral responses are mediated by the endogenous kappa opioid system. Neuropsychopharmacology 31 (6), 1241e1248. Merali, Z., Kent, P., Du, L., Hrdina, P., Palkovits, M., Faludi, G., et al., 2006. Corticotropin-releasing hormone, arginine vasopressin, gastrin-releasing peptide, and neuromedin B alterations in stress-relevant brain regions of suicides and control subjects. Biological Psychiatry 59 (7), 594e602. Moore, C.A., Milano, S.K., Benovic, J.L., 2007. Regulation of receptor trafficking by GRKs and arrestins. Annual Review of Physiology 69, 451e482. Muller, M.B., Zimmermann, S., Sillaber, I., Hagemeyer, T.P., Deussing, J.M., Timpl, P., et al., 2003. Limbic corticotropin-releasing hormone receptor 1 mediates anxiety-related behavior and hormonal adaptation to stress. Nature Neuroscience 6 (10), 1100e1107. Murgatroyd, C., Patchev, A.V., Wu, Y., Micale, V., Bockmuhl, Y., Fischer, D., et al., 2009. Dynamic DNA methylation programs persistent adverse effects of early-life stress. Nature Neuroscience 12 (12), 1559e1566. Murrough, J.W., Charney, D.S., 2017. Corticotropin-releasing factor type 1 receptor antagonists for stress-related disorders: time to call it quits? Biological Psychiatry 82 (12), 858e860. Nemeroff, C.B., Bissette, G., Akil, H., Fink, M., 1991. Neuropeptide concentrations in the cerebrospinal fluid of depressed patients treated with electroconvulsive therapy. Corticotrophin-releasing factor, beta-endorphin and somatostatin. British Journal of Psychiatry 158, 59e63.

References

253

Nemeroff, C.B., Widerlov, E., Bissette, G., Walleus, H., Karlsson, I., Eklund, K., et al., 1984. Elevated concentrations of CSF corticotropin-releasing factor-like immunoreactivity in depressed patients. Science 226 (4680), 1342e1344. Nestler, E.J., Pena, C.J., Kundakovic, M., Mitchell, A., Akbarian, S., 2016. Epigenetic basis of mental illness. The Neuroscientist 22 (5), 447e463. Paez-Pereda, M., Hausch, F., Holsboer, F., 2011. Corticotropin releasing factor receptor antagonists for major depressive disorder. Expert Opinion on Investigational Drugs 20 (4), 519e535. Page, G.G., Opp, M.R., Kozachik, S.L., 2016. Sex differences in sleep, anhedonia, and HPA axis activity in a rat model of chronic social defeat. Neurobiology Stress 3, 105e113. Pentkowski, N.S., Litvin, Y., Blanchard, D.C., Vasconcellos, A., King, L.B., Blanchard, R.J., 2009. Effects of acidicastressin and ovine-CRF microinfusions into the ventral hippocampus on defensive behaviors in rats. Hormones and Behavior 56 (1), 35e43. Perrin, M.H., Vale, W.W., 1999. Corticotropin releasing factor receptors and their ligand family. Annals of the New York Academy of Sciences 885, 312e328. Philbert, J., Belzung, C., Griebel, G., 2013. The CRF(1) receptor antagonist SSR125543 prevents stress-induced cognitive deficit associated with hippocampal dysfunction: comparison with paroxetine and D-cycloserine. Psychopharmacology (Berlin) 228 (1), 97e107. Pitman, R.K., Rasmusson, A.M., Koenen, K.C., Shin, L.M., Orr, S.P., Gilbertson, M.W., et al., 2012. Biological studies of post-traumatic stress disorder. Nature Reviews Neuroscience 13 (11), 769e787. Provencal, N., Binder, E.B., 2015. The neurobiological effects of stress as contributors to psychiatric disorders: focus on epigenetics. Current Opinion in Neurobiology 30, 31e37. Pulliam, J.V., Dawaghreh, A.M., Alema-Mensah, E., Plotsky, P.M., 2010. Social defeat stress produces prolonged alterations in acoustic startle and body weight gain in male Long Evans rats. Journal of Psychiatric Research 44 (2), 106e111. Raadsheer, F.C., Hoogendijk, W.J., Stam, F.C., Tilders, F.J., Swaab, D.F., 1994. Increased numbers of corticotropinreleasing hormone expressing neurons in the hypothalamic paraventricular nucleus of depressed patients. Neuroendocrinology 60 (4), 436e444. Raadsheer, F.C., van Heerikhuize, J.J., Lucassen, P.J., Hoogendijk, W.J., Tilders, F.J., Swaab, D.F., 1995. Corticotropinreleasing hormone mRNA levels in the paraventricular nucleus of patients with Alzheimer’s disease and depression. American Journal of Psychiatry 152 (9), 1372e1376. Rainnie, D.G., Bergeron, R., Sajdyk, T.J., Patil, M., Gehlert, D.R., Shekhar, A., 2004. Corticotrophin releasing factorinduced synaptic plasticity in the amygdala translates stress into emotional disorders. Journal of Neuroscience 24 (14), 3471e3479. Ramot, A., Jiang, Z., Tian, J.B., Nahum, T., Kuperman, Y., Justice, N., Chen, A., 2017. Hypothalamic CRFR1 is essential for HPA axis regulation following chronic stress. Nature Neuroscience 20 (3), 385e388. Rau, A.R., Chappell, A.M., Butler, T.R., Ariwodola, O.J., Weiner, J.L., 2015. Increased basolateral amygdala pyramidal cell excitability may contribute to the anxiogenic phenotype induced by chronic early-life stress. Journal of Neuroscience 35 (26), 9730e9740. Regev, L., Neufeld-Cohen, A., Tsoory, M., Kuperman, Y., Getselter, D., Gil, S., Chen, A., 2011. Prolonged and sitespecific over-expression of corticotropin-releasing factor reveals differential roles for extended amygdala nuclei in emotional regulation. Molecular Psychiatry 16 (7), 714e728. Regev, L., Tsoory, M., Gil, S., Chen, A., 2012. Site-specific genetic manipulation of amygdala corticotropin-releasing factor reveals its imperative role in mediating behavioral response to challenge. Biological Psychiatry 71 (4), 317e326. Reul, J., Holsboer, F., 2002. On the role of corticotropin-releasing hormone receptors in anxiety and depression. Dialogues in Clinical Neuroscience 4, 31e46. Reyes, B.A., Fox, K., Valentino, R.J., Van Bockstaele, E.J., 2006. Agonist-induced internalization of corticotropinreleasing factor receptors in noradrenergic neurons of the rat locus coeruleus. European Journal of Neuroscience 23 (11), 2991e2998. Reyes, B.A., Valentino, R.J., Van Bockstaele, E.J., 2008. Stress-induced intracellular trafficking of corticotropinreleasing factor receptors in rat locus coeruleus neurons. Endocrinology 149 (1), 122e130. Rice, C.J., Sandman, C.A., Lenjavi, M.R., Baram, T.Z., 2008. A novel mouse model for acute and long-lasting consequences of early life stress. Endocrinology 149 (10), 4892e4900.

254

16. The role of the CRF-urocortin system in stress resilience

Roozendaal, B., Brunson, K.L., Holloway, B.L., McGaugh, J.L., Baram, T.Z., 2002. Involvement of stress-released corticotropin-releasing hormone in the basolateral amygdala in regulating memory consolidation. Proceedings of the National Academy of Sciences of the United States of America 99 (21), 13908e13913. Roozendaal, B., McEwen, B.S., Chattarji, S., 2009. Stress, memory and the amygdala. Nature Reviews Neuroscience 10 (6), 423e433. Roozendaal, B., Schelling, G., McGaugh, J.L., 2008. Corticotropin-releasing factor in the basolateral amygdala enhances memory consolidation via an interaction with the beta-adrenoceptor-cAMP pathway: dependence on glucocorticoid receptor activation. Journal of Neuroscience 28 (26), 6642e6651. Roseboom, P.H., Nanda, S.A., Bakshi, V.P., Trentani, A., Newman, S.M., Kalin, N.H., 2007. Predator threat induces behavioral inhibition, pituitary-adrenal activation and changes in amygdala CRF-binding protein gene expression. Psychoneuroendocrinology 32 (1), 44e55. Sabariego, M., Gomez, M.J., Moron, I., Torres, C., Fernandez-Teruel, A., Tobena, A., et al., 2011. Differential gene expression between inbred Roman high- (RHA-I) and low- (RLA-I) avoidance rats. Neuroscience Letters 504 (3), 265e270. Sajdyk, T.J., Schober, D.A., Gehlert, D.R., Shekhar, A., 1999. Role of corticotropin-releasing factor and urocortin within the basolateral amygdala of rats in anxiety and panic responses. Behavioural Brain Research 100 (1e2), 207e215. Salim, S., Eikenburg, D.C., 2007. Role of 90-kDa heat shock protein (Hsp 90) and protein degradation in regulating neuronal levels of G protein-coupled receptor kinase 3. Journal of Pharmacology and Experimental Therapeutics 320 (3), 1106e1112. Sandi, C., Cordero, M.I., Ugolini, A., Varea, E., Caberlotto, L., Large, C.H., 2008. Chronic stress-induced alterations in amygdala responsiveness and behavior–modulation by trait anxiety and corticotropin-releasing factor systems. European Journal of Neuroscience 28 (9), 1836e1848. Sanders, J., Nemeroff, C., 2016. The CRF System as a Therapeutic Target for Neuropsychiatric Disorders. Trends in Pharmacological Sciences 37 (12), 1045e1054. Seasholtz, A.F., Burrows, H.L., Karolyi, I.J., Camper, S.A., 2001. Mouse models of altered CRH-binding protein expression. Peptides 22 (5), 743e751. Sheikh, H.I., Kryski, K.R., Smith, H.J., Hayden, E.P., Singh, S.M., 2013. Corticotropin-releasing hormone system polymorphisms are associated with children’s cortisol reactivity. Neuroscience 229, 1e11. Shekhar, A., Sajdyk, T.J., Gehlert, D.R., Rainnie, D.G., 2003. The amygdala, panic disorder, and cardiovascular responses. Annals of the New York Academy of Sciences 985, 308e325. Singh-Taylor, A., Molet, J., Jiang, S., Korosi, A., Bolton, J.L., Noam, Y., et al., 2018. NRSF-dependent epigenetic mechanisms contribute to programming of stress-sensitive neurons by neonatal experience, promoting resilience. Molecular Psychiatry 23 (3), 648e657. Sipahi, L., Wildman, D.E., Aiello, A.E., Koenen, K.C., Galea, S., Abbas, A., Uddin, M., 2014. Longitudinal epigenetic variation of DNA methyltransferase genes is associated with vulnerability to post-traumatic stress disorder. Psychological Medicine 44 (15), 3165e3179. Skutella, T., Probst, J.C., Renner, U., Holsboer, F., Behl, C., 1998. Corticotropin-releasing hormone receptor (type I) antisense targeting reduces anxiety. Neuroscience 85 (3), 795e805. Slater, P.G., Yarur, H.E., Gysling, K., 2016. Corticotropin-releasing factor receptors and their interacting proteins: functional consequences. Molecular Pharmacology 90 (5), 627e632. Smith, G.W., Aubry, J.M., Dellu, F., Contarino, A., Bilezikjian, L.M., Gold, L.H., et al., 1998. Corticotropin releasing factor receptor 1-deficient mice display decreased anxiety, impaired stress response, and aberrant neuroendocrine development. Neuron 20 (6), 1093e1102. Spiga, F., Lightman, S.L., Shekhar, A., Lowry, C.A., 2006. Injections of urocortin 1 into the basolateral amygdala induce anxiety-like behavior and c-Fos expression in brainstem serotonergic neurons. Neuroscience 138 (4), 1265e1276. Sterrenburg, L., Gaszner, B., Boerrigter, J., Santbergen, L., Bramini, M., Elliott, E., et al., 2011. Chronic stress induces sex-specific alterations in methylation and expression of corticotropin-releasing factor gene in the rat. PLoS One 6 (11), e28128. Suvrathan, A., Bennur, S., Ghosh, S., Tomar, A., Anilkumar, S., Chattarji, S., 2014. Stress enhances fear by forming new synapses with greater capacity for long-term potentiation in the amygdala. Philosophical Transactions of the Royal Society of London B Biological Sciences 369 (1633), 20130151.

References

255

Swiergiel, A.H., Leskov, I.L., Dunn, A.J., 2008. Effects of chronic and acute stressors and CRF on depression-like behavior in mice. Behavioural Brain Research 186 (1), 32e40. Sztainberg, Y., Kuperman, Y., Tsoory, M., Lebow, M., Chen, A., 2010. The anxiolytic effect of environmental enrichment is mediated via amygdalar CRF receptor type 1. Molecular Psychiatry 15 (9), 905e917. Taneja, M., Salim, S., Saha, K., Happe, H.K., Qutna, N., Petty, F., et al., 2011. Differential effects of inescapable stress on locus coeruleus GRK3, alpha2-adrenoceptor and CRF1 receptor levels in learned helpless and non-helpless rats: a potential link to stress resilience. Behavioural Brain Research 221 (1), 25e33. Timpl, P., Spanagel, R., Sillaber, I., Kresse, A., Reul, J.M., Stalla, G.K., et al., 1998. Impaired stress response and reduced anxiety in mice lacking a functional corticotropin-releasing hormone receptor 1. Nature Genetics 19 (2), 162e166. Tran, L., Schulkin, J., Greenwood-Van Meerveld, B., 2014. Importance of CRF receptor-mediated mechanisms of the bed nucleus of the stria terminalis in the processing of anxiety and pain. Neuropsychopharmacology 39 (11), 2633e2645. Uht, R.M., McKelvy, J.F., Harrison, R.W., Bohn, M.C., 1988. Demonstration of glucocorticoid receptor-like immunoreactivity in glucocorticoid-sensitive vasopressin and corticotropin-releasing factor neurons in the hypothalamic paraventricular nucleus. Journal of Neuroscience Research 19 (4), 405e411, 468e409. Ungless, M.A., Singh, V., Crowder, T.L., Yaka, R., Ron, D., Bonci, A., 2003. Corticotropin-releasing factor requires CRF binding protein to potentiate NMDA receptors via CRF receptor 2 in dopamine neurons. Neuron 39 (3), 401e407. Vale, W., Spiess, J., Rivier, C., Rivier, J., 1981. Characterization of a 41-residue ovine hypothalamic peptide that stimulates secretion of corticotropin and beta-endorphin. Science 213 (4514), 1394e1397. Valentino, R.J., Foote, S.L., Page, M.E., 1993. The locus coeruleus as a site for integrating corticotropin-releasing factor and noradrenergic mediation of stress responses. Annals of the New York Academy of Sciences 697, 173e188. van Bodegom, M., Homberg, J.R., Henckens, M., 2017. Modulation of the hypothalamic-pituitary-adrenal axis by early life stress exposure. Frontiers in Cellular Neuroscience 11, 87. Van Den Eede, F., Van Broeckhoven, C., Claes, S.J., 2005. Corticotropin-releasing factor-binding protein, stress and major depression. Ageing Research Reviews 4 (2), 213e239. Van Pett, K., Viau, V., Bittencourt, J.C., Chan, R.K., Li, H.Y., Arias, C., et al., 2000. Distribution of mRNAs encoding CRF receptors in brain and pituitary of rat and mouse. Journal of Comparative Neurology 428 (2), 191e212. Veith, R.C., Lewis, N., Langohr, J.I., Murburg, M.M., Ashleigh, E.A., Castillo, S., et al., 1993. Effect of desipramine on cerebrospinal fluid concentrations of corticotropin-releasing factor in human subjects. Psychiatry Research 46 (1), 1e8. Vicentini, J.E., Cespedes, I.C., Nascimento, J.O., Bittencourt, J.C., Viana, M.B., 2014. CRF type 1 receptors of the medial amygdala modulate inhibitory avoidance responses in the elevated T-maze. Hormones and Behavior 65 (3), 195e202. Vyas, A., Bernal, S., Chattarji, S., 2003. Effects of chronic stress on dendritic arborization in the central and extended amygdala. Brain Research 965 (1e2), 290e294. Vyas, A., Mitra, R., Shankaranarayana Rao, B.S., Chattarji, S., 2002. Chronic stress induces contrasting patterns of dendritic remodeling in hippocampal and amygdaloid neurons. Journal of Neuroscience 22 (15), 6810e6818. Walther, C., Caetano, F.A., Dunn, H.A., Ferguson, S.S., 2015. PDZK1/NHERF3 differentially regulates corticotropinreleasing factor receptor 1 and serotonin 2A receptor signaling and endocytosis. Cellular Signalling 27 (3), 519e531. Wan, Q., Gao, K., Rong, H., Wu, M., Wang, H., Wang, X., et al., 2014. Histone modifications of the Crhr1 gene in a rat model of depression following chronic stress. Behavioural Brain Research 271, 1e6. Wanat, M.J., Hopf, F.W., Stuber, G.D., Phillips, P.E., Bonci, A., 2008. Corticotropin-releasing factor increases mouse ventral tegmental area dopamine neuron firing through a protein kinase C-dependent enhancement of Ih. Journal of Physiology 586 (8), 2157e2170. Wang, S.S., Kamphuis, W., Huitinga, I., Zhou, J.N., Swaab, D.F., 2008. Gene expression analysis in the human hypothalamus in depression by laser microdissection and real-time PCR: the presence of multiple receptor imbalances. Molecular Psychiatry 13 (8), 786e799, 741. Wang, X.D., Chen, Y., Wolf, M., Wagner, K.V., Liebl, C., Scharf, S.H., et al., 2011a. Forebrain CRHR1 deficiency attenuates chronic stress-induced cognitive deficits and dendritic remodeling. Neurobiology of Disease 42 (3), 300e310.

256

16. The role of the CRF-urocortin system in stress resilience

Wang, X.D., Rammes, G., Kraev, I., Wolf, M., Liebl, C., Scharf, S.H., et al., 2011b. Forebrain CRF(1) modulates early-life stress-programmed cognitive deficits. Journal of Neuroscience 31 (38), 13625e13634. Wang, X.D., Su, Y.A., Wagner, K.V., Avrabos, C., Scharf, S.H., Hartmann, J., et al., 2013. Nectin-3 links CRHR1 signaling to stress-induced memory deficits and spine loss. Nature Neuroscience 16 (6), 706e713. Wasserman, D., Wasserman, J., Rozanov, V., Sokolowski, M., 2009. Depression in suicidal males: genetic risk variants in the CRHR1 gene. Genes, Brain and Behavior 8 (1), 72e79. Wells, A.M., Ridener, E., Bourbonais, C.A., Kim, W., Pantazopoulos, H., Carroll, F.I., et al., 2017. Effects of chronic social defeat stress on sleep and circadian rhythms are mitigated by kappa-opioid receptor antagonism. Journal of Neuroscience 37 (32), 7656e7668. Williams, C.L., Buchta, W.C., Riegel, A.C., 2014. CRF-R2 and the heterosynaptic regulation of VTA glutamate during reinstatement of cocaine seeking. Journal of Neuroscience 34 (31), 10402e10414. Willner, P., 2005. Chronic mild stress (CMS) revisited: consistency and behavioural-neurobiological concordance in the effects of CMS. Neuropsychobiology 52 (2), 90e110. Wood, S.K., Walker, H.E., Valentino, R.J., Bhatnagar, S., 2010. Individual differences in reactivity to social stress predict susceptibility and resilience to a depressive phenotype: role of corticotropin-releasing factor. Endocrinology 151 (4), 1795e1805. Zhou, J., Nagarkatti, P., Zhong, Y., Ginsberg, J.P., Singh, N.P., Zhang, J., Nagarkatti, M., 2014. Dysregulation in microRNA expression is associated with alterations in immune functions in combat veterans with posttraumatic stress disorder. PLoS One 9 (4), e94075. Zobel, A.W., Nickel, T., Kunzel, H.E., Ackl, N., Sonntag, A., Ising, M., Holsboer, F., 2000. Effects of the high-affinity corticotropin-releasing hormone receptor 1 antagonist R121919 in major depression: the first 20 patients treated. Journal of Psychiatric Research 34 (3), 171e181. Zorrilla, E.P., Valdez, G.R., Nozulak, J., Koob, G.F., Markou, A., 2002. Effects of antalarmin, a CRF type 1 receptor antagonist, on anxiety-like behavior and motor activation in the rat. Brain Research 952 (2), 188e199.

C H A P T E R

17

Intergenerational transmission of stress vulnerability and resilience 1

Mallory E. Bowers1, Rachel Yehuda1, 2, 3

Department of Psychiatry, Icahn School of Medicine at Mount Sinai, New York, NY, United States; 2Mental Health Care Center, James J. Peters Veterans Affairs Medical Center, Bronx, NY, United States; 3Department of Neuroscience, Icahn School of Medicine at Mount Sinai, New York, NY, United States

Introduction Severe stress exposure can precipitate long-term mental health problems, particularly in vulnerable individuals. It is now accepted that maladaptive behaviors and mental illness that may result from extreme stress exposure can be perpetuated intergenerationally. Although it is clear that familial psychopathology may have a genetic component, phenotypic alterations observed in subsequent generations could stem from trauma-mediated changes in parenting styles and other behaviors that are mirrored by offspring. Alternatively, adverse experiences in parents could be biologically transmitted to offspring via nongenetic mechanisms, subsequently altering offspring biology and/or behaviors in a way that precipitates or buffers against mental illness. Here, we examine the evidence that suggests environmental exposure to psychological trauma that occur prior to conception or during gestation can exert long-lasting phenotypic changes in offspring. To date, studies have focused primarily on stress during pregnancy, which primarily reflects an understanding of stress transmission from mothers to offspring via placental signaling. Alternatively, preconception stress is thought to be transmitted by both mothers and fathers through epigenetic changes in gametes. Recent studies also demonstrate that offspring outcomes may vary according to timing of parental exposures.

Stress Resilience https://doi.org/10.1016/B978-0-12-813983-7.00017-3

257

Copyright © 2020 Elsevier Inc. All rights reserved.

258

17. Intergenerational transmission of stress vulnerability and resilience

Foundational populations: studies of the Dutch hunger winter and holocaust survivor offspring In considering the origins of the field of intergenerational transmission of stress, studies of the Dutch Hunger Winter and Holocaust survivor offspring (HSO) have been foundational and have prompted an increasing number of animal studies that provide insight into the potential biological mechanisms of stress transmission. The Dutch Hunger Winter and Holocaust are dual instances of exposure to extreme environmental threat that were unrelated to the personal histories or behaviors of those affected. Study of the survivors of these profoundly adverse conditions, and of their offspring, are therefore devoid of the traumaassociated confounds that impact most naturalistic studies of trauma effects in humans. Observations of the effects of these exposures across generations have proved foundational in defining the range of physical and mental health sequelae that may be associated with exposure and have raised important questions regarding their mechanism(s) of transmission of these effects. The Dutch Hunger Winter refers to the 6-month period from 1944 to 45 during which German occupiers in the Netherlands imposed a food embargo that resulted in severe and widespread famine among Dutch citizens. Despite a robust posteWorld War II recovery, Dutch adults who were in utero during the famine exhibited increased prevalence of type II diabetes, cardiovascular disease, and age-associated cognitive decline (Tobi et al., 2014). Poorer health outcomes among offspring who were famine-exposed in utero, regardless of socioeconomic status, lent evidence to the once controversial Barker hypothesis that has now been expanded into a broad field of study related to the developmental origins of health and disease (DOHaD) (Hoy and Nicol, 2018). For example, offspring exposed to the famine in utero exhibit differences in IGF2 methylation compared with unexposed, same-sex siblings. This difference is specific to offspring exposed early in development, concurring with data suggesting that adverse health outcomes are exhibited primarily by offspring exposed early in gestation. Initially inspired by HSO personal accounts detailing feelings of helplessness, vicarious traumatization, and mental health symptoms (Spiegelman, 1991), there are now substantial empirical data showing that offspring of Holocaust survivors are at greater risk for developing posttraumatic stress disorder (PTSD) if one or both parents had PTSD (Solomon et al., 1988; Yehuda et al., 2008). Adult offspring of Holocaust survivors are at greater risk for metabolic syndrome, specifically as hypertension and type II diabetes, as well as depression and anxiety disorders (Flory et al., 2011; Gangi et al., 2009; Yehuda et al., 1998, 2001, 2008). As in the Dutch Hunger Winter studies, the focus has shifted to epigenetics as a potential mechanism for transmission of PTSD from parents to offspring. NR3C1 is the gene that in humans is responsible for glucocorticoid receptor (GR) transcription, and circulating levels of cortisol are determined through negative feedback regulation of cortisol at the pituitary GR. Increased pituitary GR responsiveness has been demonstrated in patients with PTSD, particularly in those with chronic PTSD, as have diminished ambient levels of cortisol. Because GR transcription is in part determined by NR3C1 promoter methylation, it was of interest to examine the respective associations of NR3C1 promoter methylation with maternal and paternal PTSD. This analysis demonstrated an interaction of maternal and paternal PTSD

Maternal versus paternal transmission

259

with respect to NR3C1 methylation. In the absence of maternal PTSD, offspring with paternal PTSD showed higher levels of NR3C1 promoter methylation, whereas those with maternal PTSD, with and without paternal PTSD, showed diminished NR3C1 methylation. Moreover, lower GR 1F promoter methylation was associated with greater postdexamethasone suppression, indicating increased GR responsiveness. These results indicate that maternal PTSD, regardless of the presence or absence of paternal PTSD, is associated not only with the expression of PTSD in Holocaust offspring but also with the biological concomitants of PTSD, namely that of increased GR responsiveness assessed using the dexamethasone suppression test or the lysozyme IC-50Dex (Yehuda et al., 2014). Further examination of the effect of paternal PTSD demonstrates an association with elevated incidence of offspring major depressive disorder (MDD), consistent with the findings of increased methylation of the GR promoter, decreased expression of the receptor, and ultimately diminished feedback glucocorticoid sensitivity with increased ambient cortisol that has been associated with MDD (Lehrner et al., 2014). Maternal PTSD, more strongly associated with offspring PTSD than paternal PTSD, is associated with greater GR sensitivity in offspring and moderates an effect of paternal PTSD on offspring NR3C1 methylation. Significantly, offspring NR3C1 methylation changes appear to be specific to parental PTSD, as parental Holocaust exposure in the absence of PTSD had no effect on offspring NR3C1 promoter methylation (Yehuda et al., 2014). Notably, however, Holocaust exposure has been shown to effect methylation of a separate hypothalamic-pituitary-adrenal (HPA) axiserelated gene, FKPB5. FKBP5 is a cochaperone of heat shock protein 90, which regulates GR sensitivity by impeding cytoplasmic transport of GR to the nucleus and thereby diminishing glucocorticoid responsivity. Whereas elevated levels of FKBP5 methylation were apparent for Holocaust survivors in comparison with age and ethnically matched controls, their offspring showed diminished FKBP5 methylation in contrast to their controls (Yehuda et al., 2015). Of further interest, parent and offspring levels of FKBP5 methylation were positively correlated in both survivor and control parent-offspring pairs. Such observations have underscored the relevance of both NR3C1 and FKBP5, genes for the GR and its cochaperone, not only as critical mediators and regulators of the stress response but also as targets for the intergenerational moderation of offspring stress preparedness and potential vulnerability.

Maternal versus paternal transmission Maternal transmission Studies of maternal intergenerational transmission have principally examined effects on offspring of maternal stress experienced during the course of gestation. The influence of maternal anxiety, depression, or other forms of mental illness on offspring outcomes has been explored, as have the effects of maternal traumatic exposures, including intimate partner violence (IPV), disasters, and other acute environmental stressors. Studies purportedly examining the effects of maternal childhood adversity on offspring are often confounded by some continuation through adolescence and adulthood of the elements contributing to the childhood disruption or neglect. Such studies have largely assessed associations between mothers and offspring across behavioral/psychological, neuroendocrine, epigenetic, and, to

260

17. Intergenerational transmission of stress vulnerability and resilience

some extent, neuroanatomical domains. Only a minority of investigations of prospective mothers and their babies segregate exposures to a specific trimester, and even fewer include mediation analyses to determine trajectories of influence on observed changes in offspring. With an appreciation of the limitations imposed by naturalistic studies in humans, we highlight illustrative examples of research on intergenerational transmission of prenatal and preconception maternal stress effects. Studies examining salivary cortisol in children of mothers who experienced high levels of anxiety and/or depression during the first trimester exhibit both higher and lower levels of basal cortisol compared with controls (Gutteling et al., 2004, 2005; Van den Bergh et al., 2008). Studies examining stress or trauma experienced in the third trimester show similarly inconsistent results, in that they observe both positive and negative associations between salivary cortisol in offspring and maternal stress (O’Connor et al., 2005; O’Donnell et al., 2013; Yehuda et al., 2005). Inconsistency in the data may stem from variability in age of offspring across studies, in that offspring tested were as young as newborns and as old as 60 years. Although the majority of these studies examine salivary cortisol, others have examined plasma or 24 h urinary cortisol. Additionally, sample collection may have occurred at various times of day (cortisol exhibits a circadian rhythm) and may have occurred at baseline or in response to provocation. Further evidence indicates that offspring outcomes may vary by gender. Given that studies of extreme stress have predominantly focused on the long-term effects on the HPA axis that precipitate dysfunction, it follows then that epigenetic studies have primarily examined modifications to genes that regulate the HPA axis. Hypermethylation of the NR3C1 promoter has been observed in offspring born to mothers who experienced prenatal stress as a result of IPV, war, and ethnic cleansing during pregnancy (Mulligan et al., 2012; Perroud et al., 2014; Radtke et al., 2011). Prenatal depression and subjective experiences of stress are also positively associated with NR3C1 DNA methylation (Braithwaite et al., 2015; Mansell et al., 2016; Oberlander et al., 2008; Palma-Gudiel et al., 2015). Hypermethylation of NR3C1 could indicate decreased transcription of GR and possibly blunted cortisol negative feedback, in line with the classical neuroendocrine depression model. At least one other study, however, has reported no effect of prenatal stress on offspring NR3C1 methylation (van der Knaap et al., 2014). Anatomically, prenatal stress is associated with a decrease in offspring cortical thickness and gray matter volume in the prefrontal cortex (Buss et al., 2010; Qiu et al., 2015). Postnatal maternal anxiety has also been associated with greater right hippocampal growth and left hippocampal volume in offspring (Pruessner et al., 2008; Qiu et al., 2013). Aside from the Yehuda Holocaust studies, very few studies have examined biologicalbehavioral outcomes in offspring born to mothers who experienced preconception stress. Stress is rarely acute enough to neatly affect a specific window of time (a confounding factor for prenatal stress studies, additionally). However, at least some data suggest that preconception stress is associated with decreased birth weight and increased infant mortality (Cheng et al., 2016; Class et al., 2013). As the prenatal stress literature has demonstrated, decreased birth weight is linked with a number of long-term health problems, such as type II diabetes.

Paternal transmission There are very few human studies that address the question of whether paternal experience is transmitted to offspring, despite the relative ease in collecting semen samplesdwhich

Hypothesized mechanisms of transmission

261

may provide data on whether epigenetic marks in gametes could act as a direct mechanism of transmission between parent and offspring. A recent study finds some evidence for specific differential DNA modifications associated with MDD that is common across white blood cells, prefrontal cortical tissue, and sperm. Tissue samples, however, were from nonoverlapping cohorts of participants (Oh et al., 2015). Changes in DNA methylation are also observed in sperm from men addicted to alcohol and in sperm from men addicted to opioids. Specifically, H19 differentially methylated region (DMR) and intergenic (IG) DMR methylation in sperm is correlated with alcohol consumption in men. These DMRs are thought to fall within the recognition sequences for DNMT enzymes, which could alter methylation patterns across neighboring genetic regions (Ouko et al., 2009). In spermderived DNA from opioid addicts, methylation was significantly increased at CpG 2 of OPRM1 promoter (Chorbov et al., 2011), which controls expression of opioid receptors. Changes in expression of could OPRM1 alter sensitivity and tolerance to opioids, potentially altering opioid intake. Although these initial studies are promising in that they support the hypothesis that experience alters epigenetic marks in gametes, they fail to address whether these marks are transmitted to offspring. Studies examining correlations between epigenetic marks in sperm as well as marks in offspring tissues will be instrumental in determining how parental experience is transmitted.

Hypothesized mechanisms of transmission In thinking about a biological mechanism that transmits stress experienced by the F0 generation (parents), to the F1 (offspring) and F2 (grandchildren) generations and beyond, epigenetic marks including DNA methylation, histone and protamine posttranslation modifications (PTMs), and noncoding RNAs are likely candidates to encode and transmit environmental memories. The epigenome is malleable and responsive to environmental exposures but robust and long-lasting in altering gene expression and, consequently, phenotypes. Based on data from developmental biology studies, preconception stress is thought to affect epigenetic marks in gametes, which are then transmitted or initiate changes in the epigenome of the zygote. For instance, sperm miRNAs and piRNAs are transmitted to the oocyte upon fertilization, suggesting a role for these RNAs in embryonic development (Kawano et al., 2012; Ostermeier et al., 2004; Smythies et al., 2014). Notably, sperm miR34c is essential for the first cleavage division in mouse zygotes (Krawetz et al., 2011; Liu et al., 2012). Maternal RNAs are also transferred from oocyte to zygote and are thought to drive epigenetic reprogramming during early development (Tadros and Lipshitz, 2009). Like ncRNAs, “imprinted genes” are transmitted to zygotes in that they retain DNA methylation signatures until the second wave of epigenetic reprogramming that occurs during primordial germ cell development (Hughes, 2014). Because imprinted genes are resistant to the normative, “first wave” of epigenetic reprogramming that occurs during early embryonic development, this may indicate that these genes represent one mechanism by which stress is transmitted (at least, to the F1 generation). Notably, imprinted genes play a significant role in development and have been implicated in developmental disorders with cognitive features, such as Angelman’s syndrome.

262

17. Intergenerational transmission of stress vulnerability and resilience

Along with DNA methylation and ncRNAs, histone and protamine PTMs are also candidate mechanisms for stress transmission across generations. In sperm, approximately 85% of histones are replaced with highly basic protamines, which facilitate increased compaction of DNA and transcriptional silencing of genes. Protamines appear to be modified by several types of PTMs, which suggests that epigenetic modification of protamines may serve multiple functions beyond DNA compaction (Brunner et al., 2014; Chirat et al., 1993). Remaining histones are thought to preserve existing modifications with eventual transfer to the zygote (Brykczynska et al., 2010; Hammoud et al., 2009; Li, 2002). Although fewer in number compared with protamines, sperm histones appear to cluster around developmentally related genes, which suggest that epigenetic modifications of sperm histones (and, subsequently, zygote histones) could impact offspring phenotypes, additionally. In the oocyte, 22% of the genome is associated with histone 3 (H3) methylation domains. This epigenetic signature is unique to oocytes. In early embryos, however, H3 methylation becomes restricted to transcription start sites (Dahl et al., 2016). Oocytes that contain low levels of KDM1 or “lysine-specific demethylase 1,” which removes H3 methylation, give rise to sickly mice that, if they develop to adulthood, exhibit excessive digging and anxiety-like behavior (Wasson et al., 2016). Contrary to preconception stress, in utero stress is thought to impact the fetus via placental epigenetics, which can influence how hormones and other signaling molecules are transmitted from the mother to the developing fetus. The most often studied placental gene, 11b-hydroxysteroid dehydrogenase type 2 (11b-HSD2), controls conversion of maternal cortisol to its inactive metabolite, cortisone, thus regulating fetal exposure to cortisol. Stress exposure could directly impact the developing fetus through an increase or decrease in maternal cortisol synthesis; however, stress could also alter the activity or expression of placental enzymes such as 11b-HSD2 via epigenetic changes, which could additionally alter fetal exposure to cortisol. Although some report decreases in placental HSD11B2 methylation in response to prenatal stress, others find increases in HSD11B2 methylation (Appleton et al., 2013; Monk et al., 2016). HSD11B2 methylation also appears to moderate an association between prenatal depression and offspring cortisol levels, where decreases in HSD11B2 methylation among depressed mothers are associated with an increase in offspring baseline cortisol (Stroud et al., 2016). RNA findings seem more consistent than methylation data in that several studies observe a negative correlation between prenatal stress and placental HSD11B2 mRNA expression (Capron et al., 2018; O’Donnell et al., 2012; Togher et al., 2017). How, exactly, the physiological experience of stress is translated into a gametic or placental epigenetic signature is yet to be determined, but hormones, exosomes, and/or biological changes in supporting tissues (e.g., testes) are considered potential mechanisms. The future of this avenue of research will likely be continued by developmental biology studies using animal models; however, human studies of sperm epigenetics also offer an exciting, noninvasive option to study the biological mechanisms of paternal stress transmission.

Intergenerational transmission of resilience Although the field of intergenerational transmission has often focused on stress in parents and offspring, emerging data are beginning to lend evidence to the hypothesis that specific

Intergenerational transmission of resilience

263

exposures can buffer stress transmission. For instance, studies suggest that healthy relationships, both between parents and offspring and parents and their families and peer groups, are protective against generational perpetuation of stress. Supportive and trusting relationships with intimate partners, high levels of maternal warmth toward children, and low levels of IPV break the cycle of abuse in women who were mistreated as children (Jaffee et al., 2013). “Secure attachments” between mothers and children, fostered by processing of trauma by mothers and supportive interpersonal relationships between mothers and other adults, are associated with decreased risk of child abuse in the subsequent generation (Leifer et al., 2004). A recent metaanalysis supports the conclusion that safe, stable, nurturing relationships buffer intergenerational continuity of child maltreatment (Schofield et al., 2013). Although clinical studies increasingly determine which factors are protective against transmission of the negative effects of stress across generations, an understanding of how these factors affect the biology of offspring is still incomplete. At least one study suggests that offspring with parental psychopathology concurrently exposed to high levels of a mitigating factor exhibit increased vagal withdrawal compared with offspring with parental psychopathology exposed to low levels of a mitigating factor (Sharp et al., 2012). Vagal withdrawal is assessed by respiratory sinus arrhythmia (RSA) changes in response to a stressor and is an indication of parasympathetic nervous system functioning (RSA refers to the variation in heartbeat frequency according to the cycle of respiration). Frequency of infant stroking, assessed via maternal self-report at 5 and 9 weeks after birth, also modifies associations between prenatal maternal depression and infant physiology and emotional reactivity, where increased maternal depression is associated with decreased vagal withdrawal and increased negative emotionality in infants in the presence of low maternal stroking, but not high maternal stroking (Sharp et al., 2012). Rodent studies provide more insight into biological mechanisms that might underlie factors that prevent stress transmission across generations. In the unpredictable maternal separation/maternal stress model (MSUS) used by the Mansuy group (see Chapter 18), pups (F1) are separated from mothers for 3 hours, once per day, from postnatal day 1e14. During maternal separation, dams are exposed to restraint stress or the forced swim test. As adults, F1 and F2 control and experimental males are bred to naïve wild-type females to generate F2 and F3 offspring. MSUS affects F1, F2, and F3 behaviors on a number of tasks that can be clustered according to their assessment of behavioral flexibility/impulsivity, cognition, sociability, and anxiety-like and depressive-like behaviors (Bohacek and Mansuy, 2015; Franklin et al., 2010, 2011; Gapp et al., 2014a, 2014b). At least some of these negative phenotypes in offspring can be reversed by environmental enrichment (EE) after weaning (and upon termination of MSUS). When F1 control and MSUS males are placed in social groups in an enriched cage from weaning until adulthood, F2 MSUS spends comparable time in the bright compartment of the light-dark box compared with F2 controls. Interestingly, EE reverses increased GR expression in the hippocampus of F2 MSUS mice, suggesting that at least some of the biological systems that underlie vulnerability to maladaptive behaviors similarly underlie resilience (Gapp et al., 2016). However, it is unknown whether resilience initiates biological processes that simply erase the signatures of stress (in the case of MSUS, increased GR methylation) or whether resilience initiates mechanisms independent of stress that generally negatively regulate GR expression, that is, do positive, resilience-promoting exposures have biological and behavioral consequences in the absence of negative exposures? At least

264

17. Intergenerational transmission of stress vulnerability and resilience

one study suggests that EE administered alone to an F0 generation of mice reduces depressive-like behavior and increases corticosterone after stress exposure in the F2 generation, supporting the hypothesis that resilience-promoting exposures initiate biological changes apart from changes that stem from trauma (Yeshurun et al., 2017).

Conclusions and future directions There is growing interest in understanding which offspring are affected, and in which ways, by parental trauma exposure. However, study of the mechanisms involved in intergenerational trauma is complicated, and there is a very large risk of oversimplification and biological reductionism. However, because epigenetic mechanisms provide a molecular bridge between heredity and environmental factors, these mechanisms are likely to provide important insights for and integration of the clinical literature on effects of parental trauma on offspring and animal literature on biological mechanisms of stress transmission. The importance of continued research in this area is the potential to contribute to an understanding of the relationships among biology, culture, and history. To the extent that we are shaped by earlier experiences, it is imperative to begin to understand how this occurs from a scientific perspective.

References Appleton, A.A., Armstrong, D.A., Lesseur, C., Lee, J., Padbury, J.F., Lester, B.M., Marsit, C.J., 2013. Patterning in placental 11-B hydroxysteroid dehydrogenase methylation according to prenatal socioeconomic adversity. PLoS One 8, e74691. Bohacek, J., Mansuy, I.M., 2015. Molecular insights into transgenerational non-genetic inheritance of acquired behaviours. Nature Reviews. Genetics 16, 641e652. Braithwaite, E.C., Kundakovic, M., Ramchandani, P.G., Murphy, S.E., Champagne, F.A., 2015. Maternal prenatal depressive symptoms predict infant NR3C1 1F and BDNF IV DNA methylation. Epigenetics 10, 408e417. Brunner, A.M., Nanni, P., Mansuy, I.M., 2014. Epigenetic marking of sperm by post-translational modification of histones and protamines. Epigenetics & Chromatin 7, 2. Brykczynska, U., Hisano, M., Erkek, S., Ramos, L., Oakeley, E.J., Roloff, T.C., Beisel, C., Schubeler, D., Stadler, M.B., Peters, A.H., 2010. Repressive and active histone methylation mark distinct promoters in human and mouse spermatozoa. Nature Structural & Molecular Biology 17, 679e687. Buss, C., Davis, E.P., Muftuler, L.T., Head, K., Sandman, C.A., 2010. High pregnancy anxiety during mid-gestation is associated with decreased gray matter density in 6-9-year-old children. Psychoneuroendocrinology 35, 141e153. Capron, L.E., Ramchandani, P.G., Glover, V., 2018. Maternal prenatal stress and placental gene expression of NR3C1 and HSD11B2: the effects of maternal ethnicity. Psychoneuroendocrinology 87, 166e172. Cheng, E.R., Park, H., Wisk, L.E., Mandell, K.C., Wakeel, F., Litzelman, K., Chatterjee, D., Witt, W.P., 2016. Examining the link between women’s exposure to stressful life events prior to conception and infant and toddler health: the role of birth weight. Journal of Epidemiology & Community Health 70, 245e252. Chirat, F., Arkhis, A., Martinage, A., Jaquinod, M., Chevaillier, P., Sautiere, P., 1993. Phosphorylation of human sperm protamines HP1 and HP2: identification of phosphorylation sites. Biochimica et Biophysica Acta 1203, 109e114. Chorbov, V.M., Todorov, A.A., Lynskey, M.T., Cicero, T.J., 2011. Elevated levels of DNA methylation at the OPRM1 promoter in blood and sperm from male opioid addicts. Journal of Opioid Management 7, 258e264. Class, Q.A., Khashan, A.S., Lichtenstein, P., Langstrom, N., D’Onofrio, B.M., 2013. Maternal stress and infant mortality: the importance of the preconception period. Psychological Science 24, 1309e1316.

References

265

Dahl, J.A., Jung, I., Aanes, H., Greggains, G.D., Manaf, A., Lerdrup, M., Li, G., Kuan, S., Li, B., Lee, A.Y., Preissl, S., Jermstad, I., Haugen, M.H., Suganthan, R., Bjoras, M., Hansen, K., Dalen, K.T., Fedorcsak, P., Ren, B., Klungland, A., 2016. Broad histone H3K4me3 domains in mouse oocytes modulate maternal-to-zygotic transition. Nature 537, 548e552. Flory, J.D., Bierer, L.M., Yehuda, R., 2011. Maternal exposure to the holocaust and health complaints in offspring. Disease Markers 30, 133e139. Franklin, T.B., Linder, N., Russig, H., Thony, B., Mansuy, I.M., 2011. Influence of early stress on social abilities and serotonergic functions across generations in mice. PLoS One 6, e21842. Franklin, T.B., Russig, H., Weiss, I.C., Graff, J., Linder, N., Michalon, A., Vizi, S., Mansuy, I.M., 2010. Epigenetic transmission of the impact of early stress across generations. Biological Psychiatry 68, 408e415. Gangi, S., Talamo, A., Ferracuti, S., 2009. The long-term effects of extreme war-related trauma on the second generation of Holocaust survivors. Violence & Victims 24, 687e700. Gapp, K., Bohacek, J., Grossmann, J., Brunner, A.M., Manuella, F., Nanni, P., Mansuy, I.M., 2016. Potential of environmental enrichment to prevent transgenerational effects of paternal trauma. Neuropsychopharmacology 41, 2749e2758. Gapp, K., Jawaid, A., Sarkies, P., Bohacek, J., Pelczar, P., Prados, J., Farinelli, L., Miska, E., Mansuy, I.M., 2014a. Implication of sperm RNAs in transgenerational inheritance of the effects of early trauma in mice. Nature Neuroscience 17, 667e669. Gapp, K., Soldado-Magraner, S., Alvarez-Sanchez, M., Bohacek, J., Vernaz, G., Shu, H., Franklin, T.B., Wolfer, D., Mansuy, I.M., 2014b. Early life stress in fathers improves behavioural flexibility in their offspring. Nature Communications 5, 5466. Gutteling, B.M., de Weerth, C., Buitelaar, J.K., 2004. Maternal prenatal stress and 4-6 year old children’s salivary cortisol concentrations pre- and post-vaccination. Stress: The International Journal on the Biology of Stress 7, 257e260. Gutteling, B.M., de Weerth, C., Buitelaar, J.K., 2005. Prenatal stress and children’s cortisol reaction to the first day of school. Psychoneuroendocrinology 30, 541e549. Hammoud, S.S., Nix, D.A., Zhang, H., Purwar, J., Carrell, D.T., Cairns, B.R., 2009. Distinctive chromatin in human sperm packages genes for embryo development. Nature 460, 473e478. Hoy, W.E., Nicol, J.L., 2018. The Barker hypothesis confirmed: association of low birth weight with all-cause natural deaths in young adult life in a remote Australian Aboriginal community. Journal of Developmental Origins of Health and Disease 1e8. Hughes, V., 2014. Sperm RNA carries marks of trauma. Nature 508, 296e297. Jaffee, S.R., Bowes, L., Ouellet-Morin, I., Fisher, H.L., Moffitt, T.E., Merrick, M.T., Arseneault, L., 2013. Safe, stable, nurturing relationships break the intergenerational cycle of abuse: a prospective nationally representative cohort of children in the United Kingdom. Journal of Adolescent Health 53, S4eS10. Kawano, M., Kawaji, H., Grandjean, V., Kiani, J., Rassoulzadegan, M., 2012. Novel small noncoding RNAs in mouse spermatozoa, zygotes and early embryos. PLoS One 7, e44542. Krawetz, S.A., Kruger, A., Lalancette, C., Tagett, R., Anton, E., Draghici, S., Diamond, M.P., 2011. A survey of small RNAs in human sperm. Human Reproduction 26, 3401e3412. Lehrner, A., Bierer, L.M., Passarelli, V., Pratchett, L.C., Flory, J.D., Bader, H.N., Harris, I.R., Bedi, A., Daskalakis, N.P., Makotkine, I., Yehuda, R., 2014. Maternal PTSD associates with greater glucocorticoid sensitivity in offspring of Holocaust survivors. Psychoneuroendocrinology 40, 213e220. Leifer, M., Kilbane, T., Jacobsen, T., Grossman, G., 2004. A three-generational study of transmission of risk for sexual abuse. Journal of Clinical Child and Adolescent Psychology 33, 662e672. Li, E., 2002. Chromatin modification and epigenetic reprogramming in mammalian development. Nature Reviews. Genetics 3, 662e673. Liu, W.M., Pang, R.T., Chiu, P.C., Wong, B.P., Lao, K., Lee, K.F., Yeung, W.S., 2012. Sperm-borne microRNA-34c is required for the first cleavage division in mouse. Proceedings of the National Academy of Sciences of the United States of America 109, 490e494. Mansell, T., Vuillermin, P., Ponsonby, A.L., Collier, F., Saffery, R., Barwon Infant Study Investigator, T., Ryan, J., 2016. Maternal mental well-being during pregnancy and glucocorticoid receptor gene promoter methylation in the neonate. Development and Psychopathology 28, 1421e1430.

266

17. Intergenerational transmission of stress vulnerability and resilience

Monk, C., Feng, T., Lee, S., Krupska, I., Champagne, F.A., Tycko, B., 2016. Distress during pregnancy: epigenetic regulation of placenta glucocorticoid-related genes and fetal neurobehavior. American Journal of Psychiatry 173, 705e713. Mulligan, C.J., D’Errico, N.C., Stees, J., Hughes, D.A., 2012. Methylation changes at NR3C1 in newborns associate with maternal prenatal stress exposure and newborn birth weight. Epigenetics 7, 853e857. O’Connor, T.G., Ben-Shlomo, Y., Heron, J., Golding, J., Adams, D., Glover, V., 2005. Prenatal anxiety predicts individual differences in cortisol in pre-adolescent children. Biological Psychiatry 58, 211e217. O’Donnell, K.J., Bugge Jensen, A., Freeman, L., Khalife, N., O’Connor, T.G., Glover, V., 2012. Maternal prenatal anxiety and downregulation of placental 11beta-HSD2. Psychoneuroendocrinology 37, 818e826. O’Donnell, K.J., Glover, V., Jenkins, J., Browne, D., Ben-Shlomo, Y., Golding, J., O’Connor, T.G., 2013. Prenatal maternal mood is associated with altered diurnal cortisol in adolescence. Psychoneuroendocrinology 38, 1630e1638. Oberlander, T.F., Weinberg, J., Papsdorf, M., Grunau, R., Misri, S., Devlin, A.M., 2008. Prenatal exposure to maternal depression, neonatal methylation of human glucocorticoid receptor gene (NR3C1) and infant cortisol stress responses. Epigenetics 3, 97e106. Oh, G., Wang, S.C., Pal, M., Chen, Z.F., Khare, T., Tochigi, M., Ng, C., Yang, Y.A., Kwan, A., Kaminsky, Z.A., Mill, J., Gunasinghe, C., Tackett, J.L., Gottesman, I.I., Willemsen, G., de Geus, E.J., Vink, J.M., Slagboom, P.E., Wray, N.R., Heath, A.C., Montgomery, G.W., Turecki, G., Martin, N.G., Boomsma, D.I., McGuffin, P., Kustra, R., Petronis, A., 2015. DNA modification study of major depressive disorder: beyond locus-by-locus comparisons. Biological Psychiatry 77, 246e255. Ostermeier, G.C., Miller, D., Huntriss, J.D., Diamond, M.P., Krawetz, S.A., 2004. Reproductive biology: delivering spermatozoan RNA to the oocyte. Nature 429, 154. Ouko, L.A., Shantikumar, K., Knezovich, J., Haycock, P., Schnugh, D.J., Ramsay, M., 2009. Effect of alcohol consumption on CpG methylation in the differentially methylated regions of H19 and IG-DMR in male gametes: implications for fetal alcohol spectrum disorders. Alcoholism: Clinical and Experimental Research 33, 1615e1627. Palma-Gudiel, H., Cordova-Palomera, A., Leza, J.C., Fananas, L., 2015. Glucocorticoid receptor gene (NR3C1) methylation processes as mediators of early adversity in stress-related disorders causality: a critical review. Neuroscience & Biobehavioral Reviews 55, 520e535. Perroud, N., Rutembesa, E., Paoloni-Giacobino, A., Mutabaruka, J., Mutesa, L., Stenz, L., Malafosse, A., Karege, F., 2014. The Tutsi genocide and transgenerational transmission of maternal stress: epigenetics and biology of the HPA axis. World Journal of Biological Psychiatry 15, 334e345. Pruessner, J.C., Dedovic, K., Khalili-Mahani, N., Engert, V., Pruessner, M., Buss, C., Renwick, R., Dagher, A., Meaney, M.J., Lupien, S., 2008. Deactivation of the limbic system during acute psychosocial stress: evidence from positron emission tomography and functional magnetic resonance imaging studies. Biological Psychiatry 63, 234e240. Qiu, A., Rifkin-Graboi, A., Chen, H., Chong, Y.S., Kwek, K., Gluckman, P.D., Fortier, M.V., Meaney, M.J., 2013. Maternal anxiety and infants’ hippocampal development: timing matters. Translational Psychiatry 3, e306. Qiu, A., Tuan, T.A., Ong, M.L., Li, Y., Chen, H., Rifkin-Graboi, A., Broekman, B.F., Kwek, K., Saw, S.M., Chong, Y.S., Gluckman, P.D., Fortier, M.V., Holbrook, J.D., Meaney, M.J., 2015. COMT haplotypes modulate associations of antenatal maternal anxiety and neonatal cortical morphology. American Journal of Psychiatry 172, 163e172. Radtke, K.M., Ruf, M., Gunter, H.M., Dohrmann, K., Schauer, M., Meyer, A., Elbert, T., 2011. Transgenerational impact of intimate partner violence on methylation in the promoter of the glucocorticoid receptor. Translational Psychiatry 1, e21. Schofield, T.J., Lee, R.D., Merrick, M.T., 2013. Safe, stable, nurturing relationships as a moderator of intergenerational continuity of child maltreatment: a meta-analysis. Journal of Adolescent Health 53, S32eS38. Sharp, H., Pickles, A., Meaney, M., Marshall, K., Tibu, F., Hill, J., 2012. Frequency of infant stroking reported by mothers moderates the effect of prenatal depression on infant behavioural and physiological outcomes. PLoS One 7, e45446. Smythies, J., Edelstein, L., Ramachandran, V., 2014. Molecular mechanisms for the inheritance of acquired characteristics-exosomes, microRNA shuttling, fear and stress: lamarck resurrected? Frontiers in Genetics 5, 133. Solomon, Z., Kotler, M., Mikulincer, M., 1988. Combat-related posttraumatic stress disorder among secondgeneration Holocaust survivors: preliminary findings. American Journal of Psychiatry 145, 865e868. Spiegelman, A., 1991. Maus II : A Survivor’s Tale : and Here My Troubles Began, first ed. Pantheon Books, New York.

References

267

Stroud, L.R., Papandonatos, G.D., Parade, S.H., Salisbury, A.L., Phipps, M.G., Lester, B.M., Padbury, J.F., Marsit, C.J., 2016. Prenatal major depressive disorder, placenta glucocorticoid and serotonergic signaling, and infant cortisol response. Psychosomatic Medicine 78, 979e990. Tadros, W., Lipshitz, H.D., 2009. The maternal-to-zygotic transition: a play in two acts. Development 136, 3033e3042. Tobi, E.W., Goeman, J.J., Monajemi, R., Gu, H., Putter, H., Zhang, Y., Slieker, R.C., Stok, A.P., Thijssen, P.E., Muller, F., van Zwet, E.W., Bock, C., Meissner, A., Lumey, L.H., Eline Slagboom, P., Heijmans, B.T., 2014. DNA methylation signatures link prenatal famine exposure to growth and metabolism. Nature Communications 5, 5592. Togher, K.L., Treacy, E., O’Keeffe, G.W., Kenny, L.C., 2017. Maternal distress in late pregnancy alters obstetric outcomes and the expression of genes important for placental glucocorticoid signalling. Psychiatry Research 255, 17e26. Van den Bergh, B.R., Van Calster, B., Smits, T., Van Huffel, S., Lagae, L., 2008. Antenatal maternal anxiety is related to HPA-axis dysregulation and self-reported depressive symptoms in adolescence: a prospective study on the fetal origins of depressed mood. Neuropsychopharmacology 33, 536e545. van der Knaap, L.J., Riese, H., Hudziak, J.J., Verbiest, M.M., Verhulst, F.C., Oldehinkel, A.J., van Oort, F.V., 2014. Glucocorticoid receptor gene (NR3C1) methylation following stressful events between birth and adolescence. The TRAILS study. Translational Psychiatry 4, e381. Wasson, J.A., Simon, A.K., Myrick, D.A., Wolf, G., Driscoll, S., Pfaff, S.L., Macfarlan, T.S., Katz, D.J., 2016. Maternally provided LSD1/KDM1A enables the maternal-to-zygotic transition and prevents defects that manifest postnatally. Elife 5. Yehuda, R., Bell, A., Bierer, L.M., Schmeidler, J., 2008. Maternal, not paternal, PTSD is related to increased risk for PTSD in offspring of Holocaust survivors. Journal of Psychiatric Research 42, 1104e1111. Yehuda, R., Daskalakis, N.P., Bierer, L.M., Bader, H.N., Klengel, T., Holsboer, F., Binder, E.B., 2016. Holocaust Exposure Induced Intergenerational Effects on FKBP5 Methylation. Biological Psychiatry 80, 372e380. Yehuda, R., Daskalakis, N.P., Lehrner, A., Desarnaud, F., Bader, H.N., Makotkine, I., Flory, J.D., Bierer, L.M., Meaney, M.J., 2014. Influences of maternal and paternal PTSD on epigenetic regulation of the glucocorticoid receptor gene in Holocaust survivor offspring. American Journal of Psychiatry 171, 872e880. Yehuda, R., Engel, S.M., Brand, S.R., Seckl, J., Marcus, S.M., Berkowitz, G.S., 2005. Transgenerational effects of posttraumatic stress disorder in babies of mothers exposed to the World Trade Center attacks during pregnancy. Journal of Clinical Endocrinology & Metabolism 90, 4115e4118. Yehuda, R., Halligan, S.L., Bierer, L.M., 2001. Relationship of parental trauma exposure and PTSD to PTSD, depressive and anxiety disorders in offspring. Journal of Psychiatric Research 35, 261e270. Yehuda, R., Schmeidler, J., Wainberg, M., Binder-Brynes, K., Duvdevani, T., 1998. Vulnerability to posttraumatic stress disorder in adult offspring of Holocaust survivors. American Journal of Psychiatry 155, 1163e1171. Yeshurun, S., Short, A.K., Bredy, T.W., Pang, T.Y., Hannan, A.J., 2017. Paternal environmental enrichment transgenerationally alters affective behavioral and neuroendocrine phenotypes. Psychoneuroendocrinology 77, 225e235.

C H A P T E R

18

Stress and its effects across generations Olivia Engmann, Isabelle M. Mansuy Laboratory of Neuroepigenetics, Brain Research Institute, Medical faculty of the University of Zurich and Institute for Neuroscience, Department of Health Science and Technology, ETH Zurich, Switzerland

What is epigenetic inheritance? Stress-related psychiatric conditions such as depression and posttraumatic stress disorder (PTSD) have a strong heritable component (Eichler et al., 2010; Kendler, 2001), but, despite many twin and genome-wide association studies, genetics has only been able to explain a minor portion of heritability (Eichler et al., 2010). This is a limiting factor in understanding and adequately treating these disorders. While the classic view of biological inheritance is based on genomic sequence, it is now recognized that epigenetic factors can underlie the transmission of phenotypes to future generations (Nagy and Turecki, 2015; Nilsson and Skinner, 2015). Here, we refer to epigenetics as mechanisms that modify the genome without affecting the DNA sequence itself. Epigenetic mechanisms are essential for complex neuronal processes such as the stress response (Zannas and West, 2014), memory formation (Jarome et al., 2014), and drug addiction (Nestler, 2014). In line with this definition, epigenetic inheritance is the transmission of environmentally induced traits to subsequent generations that depend on epigenetic mechanisms. When these mechanisms affect germ cells, inheritance depends on the germline. Increasing evidence suggests that epigenetic changes in germ cells can be transmitted, but the underlying mechanisms are still not fully understood. Transgenerational epigenetic inheritance is defined as germ celledependent transmission of functional states via factors that are sensitive to the environment and modify genome activity and phenotypes across generations without changing the DNA sequence. Such a form of inheritance exists in many animal species, including Caenorhabditis elegans (Klosin et al., 2017), Drosophila

Stress Resilience https://doi.org/10.1016/B978-0-12-813983-7.00018-5

269

Copyright © 2020 Elsevier Inc. All rights reserved.

270

18. Stress and its effects across generations

melanogaster (de Vanssay et al., 2012), Mus musculus (Cropley et al., 2016; Franklin et al., 2010; Gapp et al., 2014a), and humans (Pembrey et al., 2014). Today, mechanistic evidence comes essentially from rodents, which have a relatively short generation span and are amenable to molecular and epigenetic analyses and intervention (Boxes 18.1 and 18.2).

BOX 18.1

The molecular basis of epigenetic inheritance

Breaking the Weismann barrier Although the exact mechanisms of epigenetic germline transmission are still unclear, several molecular candidates have emerged. DNAme can be altered via extracellular signaling events and allow direct alterations on the genome without affecting its sequence. PTMs of histones and protamines (Brunner et al., 2014), histone variants, nucleosome positioning, and chromatin looping can alter chromatin structure and function in a dynamic fashion. Noncoding RNAs (ncRNAs)

and proteins (Berghof et al., 2015; Ramos et al., 2014) are factors that are not tightly linked to DNA but can nevertheless contribute to inter- or transgenerational inheritance (Fig. 18.1). They can be transferred from somatic cells to sperm via exosomes (Caballero et al., 2013; Du et al., 2016; Sharma, 2014), making them carriers of information that defy the Weismann barrier. Understanding the mechanisms underlying epigenetic inheritance can open new doors to our understanding of disease susceptibility and may provide molecular targets for

FIGURE 18.1 Possible molecular mediators of epigenetic inheritance in sperm. Sperm chromatin and genome can be modified by DNA methylation (DNAme), protamine, and histone PTMs as well as chromatinbinding RNAs and proteins. It may also be structurally altered by chromatin looping. Sperm contain many noncoding RNAs (ncRNA) and proteins that can be transferred to the oocyte upon fertilization.

What is epigenetic inheritance?

BOX 18.1 therapeutic agents to reverse the consequences of stress in previous generations. While relatively little is known about mechanisms in humans, we will summarize emerging pathways from animal models, which may be applied to patient studies in the future.

DNA methylation DNA is methylated by the addition of a methyl residue to the pyrimidine ring of cytosines. Although other residues can be methylated as well, DNAme occurs most often in CG dinucleotide sequences. DNAme directly affects DNA structure

271

(cont'd) and thereby alters the binding of DNAassociated molecules at specific loci. Consequently, DNAme is a precise and potent regulatory mechanism of gene expression and chromatin architecture, most often associated with gene silencing (Lister et al., 2013; Tost, 2010). While extensive histone replacements by protamines affect the majority of other chromatin modifications, DNAme is associated with the DNA strand itself. This makes DNAme in theory a strong candidate to mediate transgenerational epigenetic effects. However, DNAme is vastly erased during early development to ensure totipotency of the zygote (Fig. 18.2) (Feng et al., 2010;

FIGURE 18.2 Critical windows and epigenetic modifications during spermatogenesis in mice. Rodent germ cell precursors start developing shortly after fertilization. Primordial germ cells (PGCs) rapidly proliferate until late embryonic development. Shortly before birth, they give rise to prospermatogonia. During this embryonic time window, DNA methylation (DNAme) and histone posttranslational modifications (PTMs) are erased at many loci and sperm RNA is regulated. After birth, spermatogonial stem cells start differentiating into primary and secondary spermatocytes. This process begins around postnatal day (P) 8 and continues into adult life. Spermatogonial stem cells have reestablished DNAme. A second wave of synthesis of ncRNA occurs in spermatocytes. Secondary spermatocytes continually differentiate into round spermatids starting at P20. Differentiation into spermatozoa is accompanied by global histone replacement with protamines, with some genomic loci being spared. Sperm is generated through life starting around P35 in mice. Stressors have varying effects on epigenetic processes during spermatogenesis depending on the developmental stage. Continued

272

18. Stress and its effects across generations

BOX 18.1 Hackett and Surani, 2013; Smith et al., 2012). DNAme from the oocyte is retained at a higher degree than in sperm cells (Borgel et al., 2010). Shortly after fertilization, DNAme is again erased in developing germ cells through DNA demethylation and/or hydroxymethylation (Iqbal et al., 2011). Nevertheless, protective mechanisms are in place to maintain DNAme at specific loci, including retrovirus-like long terminal repeat transposable elements, which may be detrimental when demethylated (Lane et al., 2003; Ruden et al., 2008). There is accumulating evidence that environmental factors including stress can alter DNAme in offspring through the patriline. For instance, MSUS induces hypomethylation as well as hypermethylation at gene promoters, and this is associated with altered gene expression (Bohacek et al., 2015; Franklin et al., 2010; Gapp et al., 2014a). These DNAme changes persist in the sperm as well as the brain of the offspring. Furthermore, brains of pups that have been raised by poor or abusive dams show increased DNAme on the BDNF promoter as well as reduced BDNF gene expression (Fumagalli et al., 2007; Lee and Hoaken, 2007). This effect is only partially reversible by cross-fostering, suggesting that part of the phenotype is inherited via the germline (Roth et al., 2009). Although it is still unclear how certain CpG loci can be protected from erasure during spermatogenesis, these mechanisms are currently under heavy investigation. Epigenome editing using CRISPR-dCas9

(cont'd) technology in germ cells may provide causal links between DNAme, genome activity and gene expression, and offspring phenotypes (Hsu et al., 2014).

Histone posttranslational modifications Histones and their PTMs can be dynamically modulated by environmental factors (Vassoler et al., 2013). To date, 16 different mechanisms of histone PTMs have been described, including acetylation, methylation, phosphorylation, sumoylation, and ubiquitylation (Dawson and Kouzarides, 2012; Tweedie-Cullen et al., 2012). Histone PTMs are altered in the offspring of mice exposed to postnatal trauma and are associated with gene expression changes (Gapp et al., 2014b). However, the role of histone PTMs in transgenerational epigenetic inheritance remains elusive, largely due to the fact that the majority of histones are replaced by protamines to allow a tight and weight-efficient packaging of the sperm genome (Hammoud et al., 2009; Johnson et al., 2011a). After fertilization, protamines are removed and replaced by histones from the oocyte. The few remaining histones in sperm are retained at specific loci and therefore likely to play a functional role in gene regulation (Brykczynska et al., 2010; Hammoud et al., 2009; Johnson et al., 2011a; Jung et al., 2017; Puri et al., 2010). Furthermore, retained paternal histones keep their PTM profile (Carone et al., 2010; Hammoud et al., 2009). It is currently unclear how histones are kept at select loci.

What is epigenetic inheritance?

BOX 18.1

Noncoding RNAs Sperm contains up to 100 fg of RNA in rat and 10 e 400 fg in human. Some of this RNA is fragmented ribosomal RNA, which has been cleaved to prevent unintended translation (Johnson et al., 2011b). The remaining RNA species consist of mRNAs as well as ncRNAs (Jung et al., 2017; Krawetz et al., 2011; Mercer and Mattick, 2013). NcRNAs are of variable length and do not code for proteins. Several lines of evidence suggest that ncRNAs may fulfill the criteria to mediate epigenetic inheritance: first, quantity and composition of sperm ncRNAs can vary depending on environmental stimuli. Second, ncRNAs are not subjected to epigenetic reprogramming during germ cell development as is the case for DNAme (Meikar et al., 2011). Most importantly, sperm RNA is delivered to the oocyte on fertilization. Hence, it may be a direct carrier of information from one generation to the next (Ostermeier et al., 2004; Johnson et al., 2011a). Indeed, when sperm RNA from male mice that had been subjected to MSUS is injected into wild-type fertilized oocytes, behavioral and metabolic symptoms of MSUS are reproduced in mice arising from the RNA-injected eggs as well as in their progeny (Gapp et al., 2014). Oocytes contain a large population of ncRNAs; however, their role in epigenetic inheritance is not as well studied (Suh and Blelloch, 2011). There are various types of ncRNAs, which may have a role in epigenetic inheritance:

Small noncoding RNAs Small noncoding RNAs (sncRNAs) are short RNA sequences including microRNAs

273

(cont'd) (miRNAs), PIWI-interacting RNAs (piRNAs), small interfering RNAs (siRNAs), and small nucleolar RNAs (snoRNAs). They regulate transcriptional and translational processes (Ghildiyal and Zamore, 2009). One single human sperm cell can deliver approximately 24,000 short sncRNA molecules to the oocyte on fertilization (Kawano et al., 2012; Krawetz et al., 2011; Peng et al., 2012). MiRNAs directly control translation. MiRNAs can recruit the machinery for gene silencing to gene promoters (Kim et al., 2008; Morris et al., 2004; Younger and Corey, 2011). Furthermore, miRNAs might prevent the replacement of histones with protamines during the remodeling of the sperm genomic architecture (Johnson et al., 2011a). Postfertilization, miRNAs regulate the expression of key genes in the zygote and therefore impact embryonic development (Liu et al., 2012; Pang et al., 2011). Chronic stress is associated with altered miRNA in sperm (Gapp et al., 2014a; Rodgers et al., 2013). PiRNAs directly control translation and may play a role in the silencing of transposons by directing DNAme (Aravin et al., 2008; Fu and Wang, 2014; Law and Jacobsen, 2010). Over 1000 piRNA species have been detected in mature human sperm (Kawano et al., 2012; Krawetz et al., 2011); however, it has not been demonstrated whether they can be delivered to the oocyte. MSUS disregulates several piRNAs in sperm across generations (Gapp et al., 2014a). The role of siRNAs and snoRNAs in epigenetic inheritance is not well known.

Continued

274

18. Stress and its effects across generations

BOX 18.1 Long noncoding RNAs LncRNAs represent the most abundant class of functional RNA and comprise >50,000 transcripts in humans (Iyer et al., 2015). They have the unique ability to bind proteins and nucleic acid simultaneously (Schmidt and Kornfeld, 2016) and can thereby directly regulate gene expression by guiding transcription factors to specific genomic loci (Feng et al., 2006). Furthermore, lncRNAs can regulate enzymes that affect histone PTMs (Bose et al., 2017) or DNAme (Savell et al., 2016). LncRNAs are associated with embryonic development, differentiation, and metabolic control (Schmidt and Kornfeld, 2016), but given the interplay with other chromatin modifications, the causal role of lncRNAs in transgenerational epigenetic inheritance remains elusive.

Transfer RNA fragments Transfer RNA (tRNA) is best known as an adaptor molecule during protein synthesis. It was recently discovered that tRNA does not only come in its full-length form but also in the reduced length “tRNA halves” and even shorter “tRNA fragments.” They are best known in their regulation of translation efficiency in response to cellular stress (Anderson & Ivanov, 2014). tRNA fragments have been proposed to transmit signals to sperm cells (Sharma et al., 2016) and are therefore potential candidates for epigenetic inheritance.

(cont'd)

Chromatin looping The 3D microarchitecture of chromatin loops can regulate gene function by bringing distal regulatory domains into contact. Chromatin looping is associated with a variety of physiological functions including imprinting (Murrell, 2011; Zhang et al., 2011), differentiation (Battistelli et al., 2014), and circadian rhythms (Aguilar-Arnal et al., 2013) and has been implicated in mental illnesses including schizophrenia (Bharadwaj et al., 2013) and cocaine addiction (Engmann et al., 2017). CCCTC binding factor (CTCF), an abundant insulator protein with a functional role in chromatin looping, is present in sperm (Jung et al., 2017). Furthermore, nucleaseaccessible regions in sperm chromatin are rich in CTCF binding sites, suggesting either that CTCF contributes to the retention of nucleosomes or that there are mechanisms in place to keep original chromatin loops intact (Arpanahi et al., 2009). Indeed, Ciabarelli et al. recently provided evidence that 3D chromatin interactions are transgenerationally inherited and modifiable by environmental stimuli in Drosophila (Ciabrelli et al., 2017). As CTCF binding is regulated by DNAme, it is conceivable that chromatin loops aid in preserving critical DNAmeinduced architectural footprints on chromatin function, while DNAme itself undergoes erasure. In summary, the role of chromatin looping in epigenetic inheritance deserves further investigation.

What is epigenetic inheritance?

275

BOX 18.2

Critical periods Intergenerational and transgenerational effects in the offspring depend in part on the type, timing, intensity, and duration of parental exposure to environmental stimuli. Prenatal and early postnatal phases are most critical, as they are windows for the dynamic regulation of epigenetic marks and phases of intense germ cell development (Fig. 18.2) (Gapp et al., 2014c). The prenatal period is a particularly wellstudied phase. The maternal (and fetal) environment affects stress responsiveness later in the offspring’s life (Bergman et al, 2010; Cottrell and Seckl, 2009; Kapoor et al., 2006; Weinstock, 2008). Specifically, when early embryos are subjected to stress in utero, they may develop a higher stress sensitivity, an increased risk for depression, and anxietylike phenotypes as well as abnormal sexual development later in life (Davis and Sandman, 2012; Monk et al., 2016; Morgan and Bale, 2011; Watson et al., 1999). Some of these effects are mediated by epigenetic alterations in the placenta (Howerton et al., 2013; Monk et al., 2016; Mueller and Bale, 2008). Others are effectuated by hormonal exposure, including elevated plasma ACTH, glucocorticoids, opioids, and catecholamines (Challis et al., 2000). The early postnatal period is well studied in respect to behavioral intergenerational effects such as maternal care. For instance, early life exposure to stress can reduce the levels of maternal care in rodents (Boccia and Pedersen, 2001; Ivy et al., 2008), and this is

associated with epigenetic effects in offspring (Champagne and Meaney, 2006). The early postnatal period is critical for transgenerational inheritance because spermatogenesis proceeds from spermatogonial stem cells toward spermatids in this after birth and can be more easily influenced by environmental stimuli than mature sperm. For instance, receptors for hormones, cytokines, and growth factors are expressed at various stages of the spermatogenic cycle (Adeoya-Osiguwa et al., 2006; Zhou et al., 2002). Although many of these factors cannot cross the bloodetestis barrier, the barrier is not complete until postnatal day 21 in male mice, leaving spermatogenic cells during this stage of development vulnerable. Stress effects in adult life can act on mature germ cells, although usually less pronounced than responses to early life stress. However, they provide the advantage of being temporally closest to the offspring and are therefore most likely to represent the footprint of an environment, which the offspring will be born into. Periconceptually, the epigenome of oocytes can be affected by hormones and toxins such as bisphenol A (Machtinger and Orvieto, 2014). Additionally, the quality of coitus may indirectly affect maternal investment toward pups (Curley and Mashoodh, 2010). Furthermore, aging affects the stability of the sperm epigenome, so that sperm of fathers with an advanced paternal age may be more sensitive to environmental stressors (Jenkins and Carrell, 2012).

276

18. Stress and its effects across generations

Why epigenetic inheritance? The Darwinian dogma that natural selection favors organisms that are most fit to reproduce in their environment has proven correct in many cases. As DNA is considered a stable carrier of heritable information, it was thought that the environment could alter the fitness of offspring only by altering the DNA sequence itself. As an extension of this concept, August Weismann proposed in late 19th century that information about the environment originating in somatic cells cannot pass to the germline, a concept known as the Weismann barrier (Sharma, 2013). However, progress in epigenetics has revealed that while the environment can spare the DNA sequence in germ cells (with rare exceptions), chromatin modifications and associated factors can be modified and can provide a fast mechanism to adjust genome activity according to environmental stimuli. Some examples of somatic contribution to germ cells have been reported, which indeed question the Weismann barrier (Caballero et al., 2013; Du et al., 2016; Surani, 2016). Thus, rather than being simple mechanisms that modify genome activity, epigenetic factors can allow the transfer of environmental information to the offspring via the germline and thereby change their response to the environment. Hence, “epimutations” are more frequent than genetic mutations and contribute to phenotypic variation in a more rapid manner (Skinner, 2015). The ability to rapidly change is particularly relevant in regularly changing environments and helps survival by adjusting the organism to the anticipated environment. For instance, a heightened stress response increases the chance of survival in a nutrient-deprived and predator-rich environment (Storm and Lima, 2010). Similarly, insulin resistance in the offspring of starving parents may reduce the use of energy pools into body growth but deposit them as visceral fat, which can serve as an emergency fuel reserve during conditions of starvation. However, some of the changes may prove to be maladaptive, e.g., increased anxiety and social withdrawal due to altered stress responses or if the environment changes again or the exposure or event that induced the changes is no longer present during the lifetime of the offspring (Daskalakis et al., 2013). Hence, it has been proposed that in long-lived species, epigenetic adaptations integrate a combination of transgenerational and intergenerational effects or intergenerational phenotypic inertia, providing an averaged image of the environments of prior generations (Godfrey et al., 2007; Kuzawa, 2005). Although the combination of Darwinian selection with epigenetic inheritance allows for optimal environmental adaptation within an organism’s life span, it may lead to a mismatch of environmental adaptation as life expectancy in modern human societies has increased while environments can rapidly change. It is speculated that these factors may contribute to the rising prevalence of certain psychiatric illnesses (Rachdaoui and Sarkar, 2014). The notion of transgenerational epigenetic inheritance is still controversial in the field of biological psychiatry; to date, relatively few studies explore how environmental changes can alter stress resilience, i.e., a successful adaptation to stressful life events in the offspring. In the following paragraphs, we will discuss the impact of epigenetic inheritance on mental health, with a focus on general stress response in mammals.

Germline versus nonegermline transmission

277

Germline versus nonegermline transmission Germline-dependent transmission Epigenetic marks can be inherited via several routes, which most often work together and can thus be challenging to disentangle. The term “transgenerational” is reserved for phenotypes that are passed on to several successive generations and that in theory involve germline-dependent mechanisms. They can be induced by environmental factors but need to persist through gametogenesis and be maintained on fertilization. Epigenetic marks including DNA methylation (DNAme) and histone posttranslational modifications (PTMs) are massively erased during embryonic development, to confer totipotency to the embryo or allow repackaging of DNA in sperm. However, some marks are not erased, suggesting that they may be involved in epigenetic inheritance (Hackett et al., 2013). Although most studies are conducted on sperm for reasons of accessibility, quantity, and exclusion of maternal care effects, a few studies of epigenetic transmission have also been conducted in the matriline (Weiss et al., 2011). Symptoms and, in theory, associated epigenetic marks need to persist into the third generation to exclude direct environmental effect on the offspring’s germline and to be considered truly transgenerational (Fig. 18.3A,B).

Nonegermline transmission Environmentally induced traits can be transferred to the offspring by mechanisms that are independent of the germline, for instance, via social, behavioral, or physiological routes. These may be experienced pre- or postnatally and require exposure in every generation to be perpetuated (Daxinger and Whitelaw, 2012; Lim and Brunet, 2013). As these traits are transmitted from one generation directly to the next, transmission is intergenerational

FIGURE 18.3 Nongenetic transmission of phenotypes. (A). During direct transmission, stressors (yellow) can affect germ cell precursors in the embryo of a gestating female and thereby impact the F3 generation. (B). Transgenerational transmission is a germ lineedependent process that does not require direct contact with the parent, as can be demonstrated by in vitro fertilization or cross-fostering approaches. (C). Intergenerational transmission is from the parent to the direct offspring and can occur via social communication, body liquids, or odors therefore in a germ lineeindependent manner. A given stressor can act by several routes to impact the future offspring.

278

18. Stress and its effects across generations

(Fig. 18.3C). Cross-fostering studies (Caporali et al., 2015), artificial insemination (Sanabria et al., 2016), or in vitro fertilization (Dietz et al., 2011) can be used to distinguish interfrom transgenerational effects (Caporali et al., 2015; Dietz et al., 2011). Maternal care plays an important role in intergenerational transmission and has been shown to involve epigenetic factors. Its level can influence epigenetic modifications, for instance, low licking/grooming and arched-back nursing (LG-ABN) in rats increases DNAme at the promoter of the glucocorticoid receptor gene, while high LG-ABN decreases DNAme. This effect is abolished by cross-fostering (Weaver et al., 2004). Similarly, stressed rats show altered maternal care as well as altered oxytocin, prolactin, and corticosterone levels, an effect also present in the offspring (Babb et al., 2014). Additional intergenerational carriers of information include the placenta (Howerton and Bale, 2012), seminal fluid (Bromfield et al., 2014), maternal milk (Liu et al., 2014), the maternal gut microbiome (Stilling et al., 2014), maternal odors (Debiec and Sullivan, 2014), as well as intrauterine signals including hormones, nutrients, and immune factors (Ong and Muhlhausler, 2011; Todrank et al., 2011). Interestingly, paternal stress can also negatively impact maternal care (Mashoodh et al., 2012). In epigenetic inheritance studies, it is critical to exclude these factors to be able to conclude that transmission involves the germline.

Preclinical and clinical studies of inheritance of stress susceptibility Inherited effects of stress in rodents Stress in utero Gestational stress enhances adrenocorticotropic hormone (ACTH) and corticosterone responses to an acute stressor in female mice of the F2 generation. This is accompanied by gene expression changes in key molecules of the hypothalamicepituitaryeadrenal (HPA) axis, while males of the F2 generation display increased anxiety-like behavior and changes in amygdala gene expression (Grundwald and Brunton, 2015). Furthermore, it also affects broader phenotypes such as a reduction in gestational duration and sensorimotor function in the F3 generation of rats (Fig. 18.4) (Yao et al., 2014). Early life stress The best characterized model for transgenerational inheritance of early life trauma effects in rodents is a paradigm based on unpredictable maternal separation combined with unpredictable maternal stress (MSUS). In this mouse model, a dam is physically separated from her pups for 3 h per day unpredictably (P1-14) and during separation is also exposed to unpredictable stressors such as forced swimming and tube restraint (Franklin et al., 2010). Pups subjected to MSUS develop depressive-like behaviors as adults, as measured on the forced swim and sucrose consumption tests. They also have deficits in risk assessment, memory and social behaviors, and metabolic dysfunctions (Franklin et al., 2010; Franklin et al., 2011; Gapp et al., 2014a; Weiss et al., 2011; Bohacek et al., 2015). Some of these symptoms persist up to the fourth generation (van Steenwyk et al., 2018). MSUS-induced behavioral phenotypes are associated with persistent molecular changes, including alterations in stress pathway and serotonergic signaling (Franklin et al., 2011;

Preclinical and clinical studies of inheritance of stress susceptibility

279

FIGURE 18.4

Transgenerational effects of stress exposure in rodents and human. Stress affects future offspring differently depending on the time of exposure. Consequences of stress during gestational, postnatal, or adult periods of life are depicted. Red arrow: increase, blue arrow: decrease, triangle: general alteration, beh.: behavior.

Razoux et al., 2017). Transmission of MSUS-induced phenotypes can occur through both the matriline (Weiss et al., 2011) and the patriline (Franklin et al., 2010). Symptoms persist after cross-fostering, indicating that the effects are mediated by germline-dependent transmission (Franklin et al., 2010; Weiss et al., 2011). Adolescent and adult stress models Several lines of evidence suggest that stress later in life can affect behavioral and molecular phenotypes in the next generations, at least through the patriline. For instance, offspring of adult male mice that have been chronically stressed show an altered stress response of the HPA axis, and this is accompanied with gene expression changes in stress-sensitive brain areas (Rodgers et al., 2013). These mice also display reduced stress reactivity in the open field test as well as altered DNAme in the hippocampus and prefrontal cortex (Mychasiuk et al., 2013). Restraint stress in male mice can also affect glucose metabolism in the offspring (Wu et al., 2016). Similarly, repeated social stress in adolescent male mice increases anxiety-like phenotypes across two generations (Saavedra-Rodríguez and Feig, 2013). Finally, adult male mice that have undergone chronic social defeat sire offspring that are more susceptible to stressful stimuli (Dietz et al., 2011).

280

18. Stress and its effects across generations

Environmental enrichment In rodents, environmental enrichment, including social housing in a spacious cage containing running wheels and toys, has been proven beneficial for many aspects of animal health and behavior and has effects across generations (Fischer et al., 2007; Hannan, 2014; Mora, 2013, Benito et al., 2018). Further, an enriched environment has been shown to reverse some of the effects of trauma exposure. It ameliorates phenotypes of risk taking in the offspring of exposed males, suggesting that it can prevent the transmission of traumainduced behaviors (Gapp et al., 2016). The effects of enrichment on the offspring are not reversed by cross-fostering, suggesting transgenerational inheritance (Arai and Feig, 2011).

Inter- and transgenerational stress effects in humans Traumatic and stressful events are major risk factors for the development of psychiatric conditions later in life and across generations, especially if they occur early in life (Pesonen and Räikkönen, 2012). However, it can be challenging to tear apart the contributing factors, given the diversity of human genetic, social, and cultural backgrounds. Causality can only be attributed for events that affect a substantial population at a precise time. Additionally, given the novelty of the research field, many studies are based on retrospective patient reports, in particular, where several generations are assessed (Nadeau, 2009). Because of the long generation time and complexity of human social interactions, it is not yet possible to experimentally distinguish intergenerational from transgenerational effects. To do so, studies involving sperm donors or foster care institutions may provide clearer contexts in the future. In utero There is an increasing consensus that maternal stress during pregnancy is associated with adverse outcomes including altered stress responses in the offspring (Weinstock, 2008). For instance, a retrospective study of young adults, whose mothers reported strong stressors during pregnancy, showed an increased cortisol response to a psychosocial stress test (Entringer et al., 2009). Maternal stress or anxiety in late pregnancy is further associated with fearfulness, negative reactivity, and behavioral problems during infancy and childhood (Bergman et al, 2007; Davis et al., 2007; Huizink et al., 2002; O’Connor et al., 2003). A recent study of pregnant women affected by the 9/11 World Trade Center attack provides an example of an acute stressor, which affected a large, geographically homogenous cohort (Yehuda et al., 2005). Children, aged 1, of mothers who had developed PTSD as a result of the attack exhibited lower waking and evening cortisol concentrations compared with offspring from mothers who had not developed PTSD. This effect was most pronounced among children from mothers with PTSD who were exposed to the trauma during the third trimester (Yehuda et al., 2005). Whether or not these phenotypes will persist in subsequent generations remains to be demonstrated. Postnatal stress The majority of inter- or transgenerational studies of postnatal stress stem from Holocaust survivors. Events have been well documented, and a large population of survivors and their children have been or can be assessed. However, the long duration of the Holocaust makes it difficult to determine a critical period of exposure. Direct descendants of Holocaust survivors show a higher prevalence of depression and anxiety disorders (Yehuda et al., 2001).

Preclinical and clinical studies of inheritance of stress susceptibility

281

Furthermore, children of Holocaust survivors with PTSD have lower cortisol levels than the offspring of survivors without parental PTSD (Yehuda et al., 2007; Yehuda et al., 2009). Although the effects in direct ascendants are more robust, studies on the grandchildren of Holocaust survivors diverge. In one study, grandchildren were found to be overrepresented by 300% in a child psychiatry clinic population (Fossion et al., 2003). However, most studies found no difference in measures of aggression (Bachar et al., 1994), internalization, externalization, or general wellbeing (Sagi-Schwartz et al, 2008), or they even reported increased resilience to posttraumatic stress (Zerach and Solomon, 2016). Alterations in parental care up to the third generation were attributed to secondary traumatization (Scharf and Mayseless, 2011). Although the Holocaust was unprecedented in its scale, genocides, maltreatment, or abuse of large populations are prominent in recent history. For instance, it is suggested that the abuse at Native American boarding schools until the late 20th century, which separated young aboriginal children from their families and aimed to enforce a Western culture on them, led to increased stress susceptibility in the offspring (Bombay et al., 2014). Traumatization due to racial discrimination has been suggested to transgenerationally contribute to a spectrum of health risks in African Americans (Goosby and Heldbrink, 2013). Studies involving the Tutsi tribe who suffered in the Rwandan genocide of 1994 show a transmission of PTSD symptoms as well as altered HPA markers and DNAme changes in the gene for the glucocorticoid receptor to their offspring (Perroud et al., 2014). Taken together, these studies indicate that traumatic life events have widespread and long-lasting effects, not only on the present generation but on future generations as well.

Other environmental factors that may impact stress response across generations A variety of environmental factors have been shown to alter molecular and behavioral phenotypes across generations. In several cases, a spectrum of environmental factors can cause convergent symptoms across generations. Environmental insults typically occur in combination in nature, such as starvation, psychological trauma, and physical abuse during war times. It can be speculated that any of those factors alone or in combination may then result in germline alterations that would modify the response of the offspring to war-like situations. In line with this idea, different environmental factors could directly activate overlapping signaling pathways. For example, stressful experiences, endocrine disruptors, and alcohol all involve glucocorticoid signaling. The molecular and behavioral susceptibility to stress appears to be a phenotype, onto which a variety of environmental factors can converge transgenerationally (Fig. 18.5). Drugs of abuse Most inheritance studies on drug exposure explore the transgenerational effects on drug response or drug seeking (Vassoler et al., 2014; Vassoler et al., 2013; Yohn et al., 2015). This includes cocaine (Vassoler et al., 2013), opiates (Byrnes et al., 2011), ethanol (Finegersh and Homanics, 2014), and cannabis (Byrnes et al., 2011). However, some evidence suggests that stress resilience may be altered as well. For instance, fetal exposure to alcohol increases the sensitivity to stress in exposed rats and their adult offspring for up to three generations (Govorko et al., 2012), while cocaine has subtle effects on stress reactivity in the offspring (Killinger et al., 2012).

282

18. Stress and its effects across generations

FIGURE 18.5

Environmental factors and life experiences that can affect stress responses in descendants.

Relevance of studying inheritance of the effects of stress for society Current studies on epigenetic inheritance provide an incomplete picture of the underlying mechanisms and implications for mental illness. This is particularly true for human longitudinal studies. The assembly of cohorts spanning three or four subsequent generations demands excessive time and funding resources. Additionally, approaches beyond the wellestablished path of genetic inheritance frequently cause incredulity in scientists. However, specifically for the reason of novelty, it is paramount to better understand how epigenetic inheritance can contribute to disease risk. It opens new doors to explain the etiology, expression, and heritability of prevalent neuropsychiatric conditions including mood and personality disorders, which often run in families but cannot be explained by genetics alone. Notably, studies of trauma are biased toward Caucasian population, as seen in the ratio of Holocaust studies of European Jews compared with millions of other people who have suffered or are currently suffering prosecution, war, and physical abuse worldwide. Animal studies are also biased toward rodents, who are the favorite model for studying the molecular basis of behavior. However, other relevant models to our lives are farm animals, which undergo many stressors such as space restrictions, social stress, and early maternal separation, generation after generation for the food industry. Pathological molecular markers may be present in the products resulting from those animals, including meat and milk. Farm animals should therefore be included in future studies for ethical and food safety reasons. Identifying epigenetic mechanisms underlying the inheritance of stress effects might provide biomarkers for diagnosis and possibly prevention of illnesses. The understanding of how choices in lifestyle, including nutrition and living environments, may help us to promote

References

283

resilience against stress and counteract inherited disease risks. However, most molecular evidence is correlative. Virus-mediated gene transfer and CRISPR-dCas9 in vivo epigenome editing may enable us to better understand the causal role of RNAs and chromatin modifications on stress susceptibility and identify potentially druggable molecular targets. For instance, selectively targeting DNAme in vivo using a CRISPR-dCas9 coupled with the active unit of a DNA methyltransferase enables us to link DNAme, gene expression, and behavior. Additionally, editing the epigenome of germ cell precursors may give us unprecedented insight into the maintenance of epimutations across generations.

References Adeoya-Osiguwa, S.A., Gibbons, R., Fraser, L.R., 2006. Identification of functional alpha2- and beta-adrenergic receptors in mammalian spermatozoa. Human Reproduction (Oxford, England) 21 (6), 1555e1563. http://doi.org/10. 1093/humrep/del016. Aguilar-Arnal, L., Hakim, O., Patel, V.R., Baldi, P., Hager, G.L., Sassone-Corsi, P., 2013. Cycles in spatial and temporal chromosomal organization driven by the circadian clock. Nature Structural and Molecular Biology 20 (10), 1206e1213. http://doi.org/10.1038/nsmb.2667. Anderson, P., Ivanov, P., 2014. TRNA fragments in human health and disease. FEBS Letters. http://doi.org/10.1016/ j.febslet.2014.09.001. Arai, J.A., Feig, L.A., 2011. Long-lasting and transgenerational effects of an environmental enrichment on memory formation. Brain Research Bulletin. http://doi.org/10.1016/j.brainresbull.2010.11.003. Aravin, A.A., Sachidanandam, R., Bourc’his, D., Schaefer, C., Pezic, D., Toth, K.F., et al., 2008. A piRNA pathway primed by individual transposons is linked to de Novo DNA methylation in mice. Molecular Cell 31 (6), 785e799. http://doi.org/10.1016/j.molcel.2008.09.003. Arpanahi, A., Brinkworth, M., Iles, D., Krawetz, S.A., Paradowska, A., Platts, A.E., et al., 2009. Endonucleasesensitive regions of human spermatozoal chromatin are highly enriched in promoter and CTCF binding sequences. Genome Research 19 (8), 1338e1349. http://doi.org/10.1101/gr.094953.109. Babb, J.A., Carini, L.M., Spears, S.L., Nephew, B.C., 2014. Transgenerational effects of social stress on social behavior, corticosterone, oxytocin, and prolactin in rats. Hormones and Behavior 65 (4), 386e393. http://doi.org/10.1016/j. yhbeh.2014.03.005. Bachar, E., Cale, M., Eisenberg, J., Dasberg, H., 1994. Aggression expression in grandchildren of Holocaust survivorsea comparative study. Israel Journal of Psychiatry and Related Sciences 31 (1), 41e47. Battistelli, C., Busanello, A., Maione, R., 2014. Functional interplay between MyoD and CTCF in regulating longrange chromatin interactions during differentiation. Journal of Cell Science 127 (17), 3757e3767. http://doi. org/10.1242/jcs.149427. Benito, E., Kerimoglu, C., Ramachandran, B., Pena-Centeno, T., Jain, G., Stilling, R.M., Islam, M.R., Capece, V., Zhou, Q., Edbauer, D., Dean, C., Fischer, A., 2018. RNA-dependent intergenerational inheritance of enhanced synaptic plasticity after environmental enrichment. Cell Reports 23 (2), 546e554. https://doi.org/10.1016/ j.celrep.2018.03.059. Berghof, T.V.L., Van Der Klein, S.A.S., Arts, J.A.J., Parmentier, H.K., Van Der Poel, J.J., Bovenhuis, H., 2015. Genetic and non-genetic inheritance of natural antibodies binding keyhole limpet hemocyanin in a purebred layer chicken line. PLoS One 10 (6). http://doi.org/10.1371/journal.pone.0131088. Bergman, K., Sarkar, P., Glover, V., O’Connor, T.G., 2010. Maternal prenatal cortisol and infant cognitive development: moderation by infant-mother attachment. Biological Psychiatry 67 (11), 1026e1032. http://doi.org/10. 1016/j.biopsych.2010.01.002. Bergman, K., Sarkar, P., O’Connor, T.G., Modi, N., Glover, V., 2007. Maternal stress during pregnancy predicts cognitive ability and fearfulness in infancy. Journal of the American Academy of Child and Adolescent Psychiatry 46 (11), 1454e1463. http://doi.org/10.1097/chi.0b013e31814a62f6. Bharadwaj, R., Jiang, Y., Mao, W., Jakovcevski, M., Dincer, A., Krueger, W., et al., 2013. Conserved chromosome 2q31 conformations are associated with transcriptional regulation of GAD1 GABA synthesis enzyme and altered in prefrontal cortex of subjects with schizophrenia. Journal of Neuroscience 33 (29), 11839e11851. http://doi.org/ 10.1523/JNEUROSCI.1252-13.2013.

284

18. Stress and its effects across generations

Boccia, M.L., Pedersen, C.A., 2001. Brief vs. long maternal separations in infancy: contrasting relationships with adult maternal behavior and lactation levels of aggression and anxiety. Psychoneuroendocrinology 26 (7), 657e672. http://doi.org/10.1016/S0306-4530(01)00019-1. Bohacek, Farinelli, M., Mirante, O., Steiner, G., Gapp, K., Coiret, G., et al., 2015. Pathological brain plasticity and cognition in the offspring of males subjected to postnatal traumatic stress. Molecular Psychiatry 20 (5), 621e631. http://doi.org/10.1038/mp.2014.80. Bombay, A., Matheson, K., Anisman, H., 2014. The intergenerational effects of Indian Residential Schools: implications for the concept of historical trauma. Transcultural Psychiatry 51 (3), 320e338. http://doi.org/10.1177/ 1363461513503380. Borgel, J., Guibert, S., Li, Y., Chiba, H., Schübeler, D., Sasaki, H., et al., 2010. Targets and dynamics of promoter DNA methylation during early mouse development. Nature Genetics 42 (12), 1093e1100. http://doi.org/10.1038/ng.708. Bose, D.A., Donahue, G., Reinberg, D., Shiekhattar, R., Bonasio, R., Berger, S.L., 2017. RNA binding to CBP stimulates histone acetylation and transcription. Cell 168 (1e2), 135e149.e22. http://doi.org/10.1016/j.cell.2016.12.020. Bromfield, J.J., Schjenken, J.E., Chin, P.Y., Care, A.S., Jasper, M.J., Robertson, S.A., 2014. Maternal tract factors contribute to paternal seminal fluid impact on metabolic phenotype in offspring. Proceedings of the National Academy of Sciences of the United States of America 111 (6), 2200e2205. http://doi.org/10.1073/pnas.1305609111. Brunner, A.M., Nanni, P., Mansuy, I.M., 2014. Epigenetic marking of sperm by post-translational modification of histones and protamines. Epigenetics and Chromatin 7 (1), 2. http://doi.org/10.1186/1756-8935-7-2. Brykczynska, U., Hisano, M., Erkek, S., Ramos, L., Oakeley, E.J., Roloff, T.C., et al., 2010. Repressive and active histone methylation mark distinct promoters in human and mouse spermatozoa. Nature Structural and Molecular Biology 17 (6), 679e687. http://doi.org/10.1038/nsmb.1821. Byrnes, J., Babb, A., Scanlan, F., Byrnes, M., 2011. Adolescent opioid exposure in female rats: transgenerational effects on morphine analgesia and anxiety-like behavior in adult offspring. Behavioural Brain Research 218 (1), 200. Caballero, J.N., Frenette, G., Belleannée, C., Sullivan, R., 2013. CD9-Positive microvesicles mediate the transfer of molecules to bovine spermatozoa during epididymal maturation. PLoS One 8 (6). http://doi.org/10.1371/journal. pone.0065364. Caporali, P., Cutuli, D., Gelfo, F., Laricchiuta, D., Foti, F., De Bartolo, P., et al., 2015. Interaction does count: a crossfostering study on transgenerational effects of pre-reproductive maternal enrichment. Frontiers in Behavioral Neuroscience 9 (December), 1e12. http://doi.org/10.3389/fnbeh.2015.00320. Carone, B.R., Fauquier, L., Habib, N., Shea, J.M., Hart, C.E., Li, R., et al., 2010. Paternally induced transgenerational environmental reprogramming of metabolic gene expression in mammals. Cell 143 (7), 1084e1096. http://doi. org/10.1016/j.cell.2010.12.008. Challis, J.R.G., Matthews, S.G., Gibb, W., Lye, S.J., 2000. Endocrine and paracrine regulation of birth at term and preterm. Endocrine Reviews. http://doi.org/10.1210/edrv.21.5.0407. Champagne, F.A., Meaney, M.J., 2006. Stress during gestation alters postpartum maternal care and the development of the offspring in a rodent model. Biological Psychiatry 59 (12), 1227e1235. http://doi.org/10.1016/j.biopsych. 2005.10.016. Ciabrelli, F., Comoglio, F., Fellous, S., Bonev, B., Ninova, M., Szabo, Q., et al., 2017. Stable polycomb-dependent transgenerational inheritance of chromatin states in Drosophila. Nature Genetics. http://doi.org/10.1038/ng.3848. Cottrell, E.C., Seckl, J.R., 2009. Prenatal stress, glucocorticoids and the programming of adult disease. Frontiers in Behavioral Neuroscience 3, 19. http://doi.org/10.3389/neuro.08.019.2009. Cropley, J.E., Eaton, S.A., Aiken, A., Young, P.E., Giannoulatou, E., Ho, J.W.K., et al., 2016. Male-lineage transmission of an acquired metabolic phenotype induced by grand-paternal obesity. Molecular Metabolism 5 (8), 699e708. http://doi.org/10.1016/j.molmet.2016.06.008. Curley, J.P., Mashoodh, R., 2010. Parent-of-origin and trans-generational germline influences on behavioral development: the interacting roles of mothers, fathers, and grandparents. Developmental Psychobiology 52 (4), 312e330. http://doi.org/10.1002/dev.20430. Daskalakis, N.P., Bagot, R.C., Parker, K.J., Vinkers, C.H., de Kloet, E.R., 2013. The three-hit concept of vulnerability and resilience: toward understanding adaptation to early-life adversity outcome. Psychoneuroendocrinology 38 (9), 1858e1873. http://doi.org/10.1016/j.psyneuen.2013.06.008. Davis, E.P., Glynn, L.M., Schetter, C.D., Hobel, C., Chicz-Demet, A., Sandman, C.A., 2007. Prenatal exposure to maternal depression and cortisol influences infant temperament. Journal of the American Academy of Child and Adolescent Psychiatry 46 (6), 737e746. http://doi.org/10.1097/chi.0b013e318047b775.

References

285

Davis, E.P., Sandman, C.A., 2012. Prenatal psychobiological predictors of anxiety risk in preadolescent children. Psychoneuroendocrinology 37 (8), 1224e1233. http://doi.org/10.1016/j.psyneuen.2011.12.016. Dawson, M.A., Kouzarides, T., 2012. Cancer epigenetics: from mechanism to therapy. Cell. http://doi.org/10.1016/j. cell.2012.06.013. Daxinger, L., Whitelaw, E., 2012. Understanding transgenerational epigenetic inheritance via the gametes in mammals. Nature Reviews Genetics 13 (3), 153e162. http://doi.org/10.1038/nrg3188. de Vanssay, A., Bougé, A.-L., Boivin, A., Hermant, C., Teysset, L., Delmarre, V., et al., 2012. Paramutation in Drosophila linked to emergence of a piRNA-producing locus. Nature 490 (7418), 112e115. http://doi.org/10. 1038/nature11416. Debiec, J., Sullivan, R.M., 2014. Intergenerational transmission of emotional trauma through amygdala dependent mother-to-infant transfer of specific fear. Proceedings of the National Academy of Sciences of the United States of America 111, 12222e12227. Dietz, D.M., Laplant, Q., Watts, E.L., Hodes, G.E., Russo, S.J., Feng, J., et al., 2011. Paternal transmission of stressinduced pathologies. Biological Psychiatry 70 (5), 408e414. http://doi.org/10.1016/j.biopsych.2011.05.005. Du, J., Shen, J., Wang, Y., Pan, C., Pang, W., Diao, H., et al., 2016. Boar seminal plasma exosomes maintain sperm function by infiltrating into the sperm membrane. Oncotarget 5 (0). http://doi.org/10.18632/oncotarget.11315. Eichler, E.E., Flint, J., Gibson, G., Kong, A., Leal, S.M., Moore, J.H., Nadeau, J.H., 2010. Missing heritability and strategies for finding the underlying causes of complex disease. Nature Reviews Genetics 11 (6), 446e450. http://doi. org/10.1038/nrg2809. Engmann, O., Labonte, B., Mitchell, A., Bashtrykov, P., Calipari, E., Rosenbluh, C., et al., 2017. The largest number of cocaine-induced changes in chromatin modifications are associated with increased expression and 3D looping of Auts2. Biological Psychiatry. Engmann, O., Labonté, B., Mitchell, A., Bashtrykov, P., Calipari, E.S., Rosenbluh, C., Loh, Y.E., Walker, D.M., Burek, D., Hamilton, P.J., Issler, O., Neve, R.L., Turecki, G., Hurd, Y., Chess, A., Shen, L., Mansuy, I., Jeltsch, A., Akbarian, S., Nestler, E.J., 2017 Dec 1. Cocaine-Induced Chromatin Modifications Associate With Increased Expression and Three-Dimensional Looping of Auts2. Biol Psychiatry 82 (11), 794e805. https:// doi.org/10.1016/j.biopsych.2017.04.013. Epub 2017 May 5. PubMed PMID: 28577753; PubMed Central PMCID: PMC5671915. Entringer, S., Kumsta, R., Hellhammer, D.H., Wadhwa, P.D., West, S., 2009. Prenatal exposure to maternal psychosocial stress and HPA axis regulation in young adults. Hormones and Behavior 55 (2), 292e298. http://doi.org/ 10.1016/j.yhbeh.2008.11.006. Feng, J., Bi, C., Clark, B.S., Mady, R., Shah, P., Kohtz, J.D., 2006. The Evf-2 noncoding RNA is transcribed from the Dlx-5/6 ultraconserved region and functions as a Dlx-2 transcriptional coactivator. Genes and Development 20 (11), 1470e1484. http://doi.org/10.1101/gad.1416106. Feng, Jacobsen, S.E., Reik, W., 2010. Epigenetic Reprogramming in Plant and Animal Development, vol. 330. Science, New York, NY), pp. 622e627 (6004). http://doi.org/10.1126/science.1190614. Finegersh, A., Homanics, G.E., 2014. Paternal alcohol exposure reduces alcohol drinking and increases behavioral sensitivity to alcohol selectively in male offspring. PLoS One 9 (6). http://doi.org/10.1371/journal.pone.0099078. Fischer, A., Sananbenesi, F., Wang, X., Dobbin, M., Tsai, L.-H., 2007. Recovery of learning and memory is associated with chromatin remodelling. Nature 447 (7141), 178e182. http://doi.org/10.1038/nature05772. Fossion, P., Rejas, M.C., Servais, L., Pelc, I., Hirsch, S., 2003. Family approach with grandchildren of holocaust survivors. American Journal of Psychotherapy 57 (4), 519e527. Franklin, T.B., Linder, N., Russig, H., Thöny, B., Mansuy, I.M., 2011. Influence of early stress on social abilities and serotonergic functions across generations in mice. PLoS One 6 (7), e21842. http://doi.org/10.1371/journal.pone.0021842. Franklin, T.B., Russig, H., Weiss, I.C., Grff, J., Linder, N., Michalon, A., et al., 2010. Epigenetic transmission of the impact of early stress across generations. Biological Psychiatry 68 (5), 408e415. http://doi.org/10.1016/j. biopsych.2010.05.036. Fu, Q., Wang, P.J., 2014. Mammalian piRNAs. Spermatogenesis 4 (1), e27889. http://doi.org/10.4161/spmg.27889. Fumagalli, F., Molteni, R., Racagni, G., Riva, M.A., 2007. Stress during development: impact on neuroplasticity and relevance to psychopathology. Progress in Neurobiology 81 (4), 197e217. http://doi.org/10.1016/j.pneurobio. 2007.01.002. Gapp, K., Bohacek, J., Grossmann, J., Brunner, A., Manuella, F., Nanni, P., Mansuy, I., 2016. Potential of Environmental Enrichment to Prevent Transgenerational Effects of Paternal Trauma. Neuropsychopharmacology. [Epub ahea.

286

18. Stress and its effects across generations

Gapp, K., Jawaid, A., Sarkies, P., Bohacek, J., Pelczar, P., Prados, J., et al., 2014a. Implication of sperm RNAs in transgenerational inheritance of the effects of early trauma in mice. Nature Neuroscience 17 (5), 667e669. http://doi. org/10.1038/nn.3695. Gapp, K., Soldado-Magraner, S., Alvarez-Sánchez, M., Bohacek, J., Vernaz, G., Shu, H., et al., 2014b. Early life stress in fathers improves behavioural flexibility in their offspring. Nature Communications 5 (November), 5466. http:// doi.org/10.1038/ncomms6466. Gapp, K., von Ziegler, L., Tweedie-Cullen, R.Y., Mansuy, I.M., 2014c. Early life epigenetic programming and transmission of stress-induced traits in mammals: how and when can environmental factors influence traits and their transgenerational inheritance? BioEssays 36 (5), 491e502. http://doi.org/10.1002/bies.201300116. Ghildiyal, M., Zamore, P.D., 2009. Small silencing RNAs: an expanding universe. Nature Reviews Genetics 10 (2), 94e108. http://doi.org/10.1038/nrg2504. Godfrey, K.M., Lillycrop, K.A., Burdge, G.C., Gluckman, P.D., Hanson, M.A., 2007. Epigenetic mechanisms and the mismatch concept of the developmental origins of health and disease. Pediatric Research. http://doi.org/10. 1203/pdr.0b013e318045bedb. Goosby, B., Heldbrink, C., 2013. Transgenerational consequences of racial discrimination for African American Health. Sociol Compass 7 (8), 630e643. http://doi.org/10.1111/soc4.12054. Govorko, D., Bekdash, R. a, Zhang, C., Sarkar, D.K., 2012. Male germline transmits fetal alcohol adverse effect on hypothalamic proopiomelanocortin gene across generations. Biological Psychiatry 72 (5), 378e388. http://doi. org/10.1016/j.biopsych.2012.04.006. Grundwald, N.J., Brunton, P.J., 2015. Prenatal stress programs neuroendocrine stress responses and affective behaviors in second generation rats in a sex-dependent manner. Psychoneuroendocrinology 62, 204e216. http://doi. org/10.1016/j.psyneuen.2015.08.010. Hackett, J.A., Sengupta, R., Zylicz, J.J., Murakami, K., Lee, C., Down, T.A., Surani, M.A., 2013. Germline DNA demethylation dynamics and imprint erasure through 5-hydroxymethylcytosine. Science 339 (6118), 448e452. http:// doi.org/10.1126/science.1229277. Hackett, J.A., Surani, M.A., 2013. Beyond DNA: programming and inheritance of parental methylomes. Cell 153 (4), 737e739. http://doi.org/10.1016/j.cell.2013.04.044. Hammoud, S.S., Nix, D.A., Zhang, H., Purwar, J., Carrell, D.T., Cairns, B.R., 2009. Distinctive chromatin in human sperm packages genes for embryo development. Nature 460 (7254), 473e478. http://doi.org/10.1038/nature08162. Hannan, a J., 2014. Environmental enrichment and brain repair: harnessing the therapeutic effects of cognitive stimulation and physical activity to enhance experience-dependent plasticity. Neuropathology and Applied Neurobiology 40 (1), 13e25. http://doi.org/10.1111/nan.12102. Howerton, C.L., Bale, T.L., 2012. Prenatal programing: at the intersection of maternal stress and immune activation. Hormones and Behavior. http://doi.org/10.1016/j.yhbeh.2012.03.007. Howerton, C.L., Morgan, C.P., Fischer, D.B., Bale, T.L., 2013. O-GlcNAc transferase (OGT) as a placental biomarker of maternal stress and reprogramming of CNS gene transcription in development. Proceedings of the National Academy of Sciences of the United States of America 110 (13), 5169e5174. http://doi.org/10.1073/pnas.1300065110. Hsu, P.D., Lander, E.S., Zhang, F., 2014. Development and applications of CRISPR-Cas9 for genome engineering. Cell 157 (6), 1262e1278. http://doi.org/10.1016/j.cell.2014.05.010. Huizink, A., Robles de Medina, P., Mulder, E., Visser, G., Buitelaar, J., 2002. Psychological measures of prenatal stress as predictors of infant temperament. Journal of the American Academy of Child and Adolescent Psychiatry 41 (9), 1078e1085. http://doi.org/10.1097/00004583-200209000-00008. Iqbal, K., Jin, S.-G., Pfeifer, G.P., Szabó, P.E., 2011. Reprogramming of the paternal genome upon fertilization involves genome-wide oxidation of 5-methylcytosine. Proceedings of the National Academy of Sciences of the United States of America 108 (9), 3642e3647. http://doi.org/10.1073/pnas.1014033108. Ivy, A.S., Brunson, K.L., Sandman, C., Baram, T.Z., 2008. Dysfunctional nurturing behavior in rat dams with limited access to nesting material: a clinically relevant model for early-life stress. Neuroscience 154 (3), 1132e1142. http://doi.org/10.1016/j.neuroscience.2008.04.019. Iyer, M., Niknafs, Y., Malik, R., Singhal, U., Sahu, A., Hosono, Y., et al., 2015. The landscape of long noncoding RNAs in the human transcriptome. Nature Genetics 47 (3), 199e208. http://doi.org/10.1038/ng.3192. Jarome, T.J., Thomas, J.S., Lubin, F.D., 2014. The epigenetic basis of memory formation and storage. Progress in Molecular Biology and Translational Science 128 (C), 1e27. http://doi.org/10.1016/B978-0-12-800977-2.00001-2. Jenkins, T.G., Carrell, D.T., 2012. The sperm epigenome and potential implications for the developing embryo. Reproduction. http://doi.org/10.1530/REP-11-0450.

References

287

Johnson, Lalancette, C., Linnemann, A.K., Leduc, F., Boissonneault, G., Krawetz, S.A., 2011a. The sperm nucleus: chromatin, RNA, and the nuclear matrix. Reproduction. http://doi.org/10.1530/REP-10-0322. Johnson, Sendler, E., Lalancette, C., Hauser, R., Diamond, M.P., Krawetz, S.A., 2011b. Cleavage of rRNA ensures translational cessation in sperm at fertilization. Molecular Human Reproduction 17 (12), 721e726. http://doi. org/10.1093/molehr/gar054. Jung, Y., Sauria, M., Lyu, X., Cheema, M., Ausio, J., Taylor, J., Corces, V., 2017. Chromatin states in mouse sperm correlate with embryonic and adult regulatory landscapes. Cell Reports 18, 1366e1382. Kapoor, A., Dunn, E., Kostaki, A., Andrews, M.H., Matthews, S.G., 2006. Fetal programming of hypothalamopituitary-adrenal function: prenatal stress and glucocorticoids. Journal of Physiology 572, 31e44. http://doi. org/10.1016/j.poly.2005.06.060. Kawano, M., Kawaji, H., Grandjean, V., Kiani, J., Rassoulzadegan, M., 2012. Novel small noncoding RNAs in mouse spermatozoa, zygotes and early embryos. PLoS One 7 (9). http://doi.org/10.1371/journal.pone.0044542. Kendler, K.S., 2001. Twin studies of psychiatric illness: an update. Archives of General Psychiatry 58 (11), 1005e1014. http://doi.org/10.1001/archpsyc.58.11.1005. Killinger, C.E., Robinson, S., Stanwood, G.D., 2012. Subtle biobehavioral effects produced by paternal cocaine exposure. Synapse 66 (10), 902e908. http://doi.org/10.1002/syn.21582. Kim, D.H., Saetrom, P., Snøve, O., Rossi, J.J., 2008. MicroRNA-directed transcriptional gene silencing in mammalian cells. Proceedings of the National Academy of Sciences of the United States of America 105 (42), 16230e16235. http://doi.org/10.1073/pnas.0808830105. Klosin, A., Casas, E., Hidalgo-Carcedo, C., Vavouri, T., Lehner, B., 2017. Transgenerational transmission of environmental information in C. elegans. Science (New York, N.Y.) 356 (6335), 320e323. http://doi.org/10.1126/science. aah6412. Krawetz, S.A., Kruger, A., Lalancette, C., Tagett, R., Anton, E., Draghici, S., Diamond, M.P., 2011. A survey of small RNAs in human sperm. Human Reproduction (Oxford, England) 26 (12), 3401e3412. http://doi.org/10.1093/ humrep/der329. Kuzawa, C.W., 2005. Fetal origins of developmental plasticity: are fetal cues reliable predictors of future nutritional environments? American Journal of Human Biology. http://doi.org/10.1002/ajhb.20091. Lane, N., Dean, W., Erhardt, S., Hajkova, P., Surani, A., Walter, J., Reik, W., 2003. Resistance of IAPs to methylation reprogramming may provide a mechanism for epigenetic inheritance in the mouse. Genesis 35 (2), 88e93. http:// doi.org/10.1002/gene.10168. Law, J.A., Jacobsen, S.E., 2010. Establishing, maintaining and modifying DNA methylation patterns in plants and animals. Nature Reviews Genetics 11 (3), 204e220. http://doi.org/10.1038/nrg2719. Lee, V., Hoaken, P.N.S., 2007. Cognition, emotion, and neurobiological development: mediating the relation between maltreatment and aggression. Child Maltreatment 12 (3), 281e298. http://doi.org/10.1177/1077559507303778. Lim, J.P., Brunet, A., 2013. Bridging the transgenerational gap with epigenetic memory. Trends in Genetics 29 (3), 176e186. http://doi.org/https://doi.org/10.1016/j.tig.2012.12.008. Lister, R., Mukamel, E. a, Nery, J.R., Urich, M., Puddifoot, C. a, Johnson, N.D., et al., 2013. Global epigenomic reconfiguration during mammalian brain development. Science (New York, N.Y.) 341 (6146), 1237905. http://doi.org/ 10.1126/science.1237905. Liu, B., Zupan, B., Laird, E., Klein, S., Gleason, G., Bozinoski, M., et al., 2014. Maternal hematopoietic TNF, via milk chemokines, programs hippocampal development and memory. Nature Neuroscience 17 (1), 97e105. http://doi. org/10.1038/nn.3596. Liu, Pang, R.T.K., Chiu, P.C.N., Wong, B.P.C., Lao, K., Lee, K.-F., Yeung, W.S.B., 2012. Sperm-borne microRNA-34c is required for the first cleavage division in mouse. Proceedings of the National Academy of Sciences of the United States of America 109 (2), 490e494. http://doi.org/10.1073/pnas.1110368109. Machtinger, R., Orvieto, R., 2014. Bisphenol A, oocyte maturation, implantation, and IVF outcome: review of animal and human data. Reproductive BioMedicine Online. http://doi.org/10.1016/j.rbmo.2014.06.013. Mashoodh, R., Franks, B., Curley, J.P., Champagne, F. a., 2012. Paternal social enrichment effects on maternal behavior and offspring growth. Proceedings of the National Academy of Sciences 109 (Suppl. ment_2), 17232e17238. http://doi.org/10.1073/pnas.1121083109. Meikar, O., Da Ros, M., Korhonen, H., Kotaja, N., 2011. Chromatoid body and small RNAs in male germ cells. Reproduction. http://doi.org/10.1530/REP-11-0057. Mercer, T.R., Mattick, J.S., 2013. Structure and function of long noncoding RNAs in epigenetic regulation. Nature Structural and Molecular Biology 20 (3), 300e307. http://doi.org/10.1038/nsmb.2480; 10.1038/nsmb.2480.

288

18. Stress and its effects across generations

Monk, C., Feng, T., Lee, S., Krupska, I., Champagne, F.A., Tycko, B., 2016. Distress during pregnancy: epigenetic regulation of placenta glucocorticoid-related genes and fetal neurobehavior. American Journal of Psychiatry 173 (7), 705e713. http://doi.org/10.1176/appi.ajp.2015.15091171. Mora, F., 2013. Successful brain aging: plasticity, environmental enrichment, and lifestyle. Dialogues in Clinical Neuroscience 15 (1), 45e52. Retrieved from: http://www.ncbi.nlm.nih.gov/pubmed/23576888%5Cnhttp:// www.pubmedcentral.nih.gov/articlerender.fcgi?artid¼PMC3622468. Morgan, C.P., Bale, T.L., 2011. Early prenatal stress epigenetically programs dysmasculinization in second-generation offspring via the paternal lineage. Journal of Neuroscience 31 (33), 11748e11755. http://doi.org/10.1523/ JNEUROSCI.1887-11.2011. Morris, K.V., Chan, S.W.-L., Jacobsen, S.E., Looney, D.J., 2004. Small interfering RNA-induced transcriptional gene silencing in human cells. Science (New York, N.Y.) 305 (5688), 1289e1292. http://doi.org/10.1126/science.1101372. Mueller, B.R., Bale, T.L., 2008. Sex-specific programming of offspring emotionality after stress early in pregnancy. Journal of Neuroscience 28 (36), 9055e9065. http://doi.org/10.1523/JNEUROSCI.1424-08.2008. Murrell, A., 2011. Setting up and maintaining differential insulators and boundaries for genomic imprinting. Biochemistry and Cell Biology 89 (5), 469e478. http://doi.org/10.1139/o11-043. Mychasiuk, R., Harker, A., Ilnytskyy, S., Gibb, R., 2013. Paternal stress prior to conception alters DNA methylation and behaviour of developing rat offspring. Neuroscience 241, 100e105. http://doi.org/10.1016/j.neuroscience. 2013.03.025. Nadeau, J.H., 2009. Transgenerational genetic effects on phenotypic variation and disease risk. Human Molecular Genetics 18 (R2). http://doi.org/10.1093/hmg/ddp366. Nagy, C., Turecki, G., 2015. Transgenerational epigenetic inheritance: an open discussion. Epigenomics 7 (5), 781e790. http://doi.org/10.2217/epi.15.46. Nestler, E.J., 2014. Epigenetic mechanisms of drug addiction. Neuropharmacology. http://doi.org/10.1016/j. neuropharm.2013.04.004. Nilsson, E.E., Skinner, M.K., 2015. Environmentally induced epigenetic transgenerational inheritance of disease susceptibility. Translational Research: The Journal of Laboratory and Clinical Medicine 165 (1), 12e17. http://doi. org/10.1016/j.trsl.2014.02.003. O’Connor, T.G., Heron, J., Golding, J., Glover, V., 2003. Maternal antenatal anxiety and behavioural/emotional problems in children: a test of a programming hypothesis. The Journal of Child Psychology and Psychiatry and Allied Disciplines 44 (7), 1025e1036. http://doi.org/10.1111/1469-7610.00187. Ong, Z.Y., Muhlhausler, B.S., 2011. Maternal “junk-food” feeding of rat dams alters food choices and development of the mesolimbic reward pathway in the offspring. The FASEB Journal: Official Publication of the Federation of American Societies for Experimental Biology 25 (7), 2167e2179. http://doi.org/10.1096/fj.10-178392. Ostermeier, G.C., Miller, D., Huntriss, J.D., Diamond, M.P., Krawetz, S.A., 2004. Reproductive biology: delivering spermatozoan RNA to the oocyte. Nature 429 (6988), 154. http://doi.org/10.1038/429154a. Pang, R.T.K., Liu, W.M., Leung, C.O.N., Ye, T.M., Kwan, P.C.K., Lee, K.F., Yeung, W.S.B., 2011. miR-135a regulates preimplantation embryo development through down-regulation of E3 ubiquitin ligase seven in absentia homolog 1A (SIAH1A) expression. PLoS One 6 (11). http://doi.org/10.1371/journal.pone.0027878. Pembrey, M., Saffery, R., Bygren, L.O., 2014. Human transgenerational responses to early-life experience: potential impact on development, health and biomedical research. Journal of Medical Genetics 51 (9), 563e572. http:// doi.org/10.1136/jmedgenet-2014-102577. Peng, H., Shi, J., Zhang, Y., Zhang, H., Liao, S., Li, W., et al., 2012. A novel class of tRNA-derived small RNAs extremely enriched in mature mouse sperm. Cell Research 22 (11), 1609e1612. http://doi.org/10.1038/cr.2012.141. Perroud, N., Rutembesa, E., Paoloni-Giacobino, A., Mutabaruka, J., Mutesa, L., Stenz, L., et al., 2014. The Tutsi genocide and transgenerational transmission of maternal stress: epigenetics and biology of the HPA axis. World Journal of Biological Psychiatry 15 (4), 334e345. http://doi.org/10.3109/15622975.2013.866693. Pesonen, A.-K., Räikkönen, K., 2012. The lifespan consequences of early life stress. Physiology and Behavior 106 (5), 722e727. http://doi.org/10.1016/j.physbeh.2011.10.030. Puri, D., Dhawan, J., Mishra, R.K., 2010. The paternal hidden agenda: epigenetic inheritance through sperm chromatin. Epigenetics: Official Journal of the DNA Methylation Society. http://doi.org/10.4161/epi.5.5.12005. Rachdaoui, N., Sarkar, D.K., 2014. Transgenerational epigenetics and brain disorders. International Review of Neurobiology 115, 51e73. http://doi.org/10.1016/B978-0-12-801311-3.00002-0. Ramos, R., Garnier, R., González-Solís, J., Boulinier, T., 2014. Long antibody persistence and transgenerational transfer of immunity in a long-lived vertebrate. The American Naturalist 184 (6), 764e776. http://doi.org/10.1086/678400.

References

289

Razoux, F., Russig, H., Mueggler, T., Baltes, C., Dikaiou, K., Rudin, M., Mansuy, I.M., 2017. Transgenerational disruption of functional 5-HT1AR-induced connectivity in the adult mouse brain by traumatic stress in early life. Molecular Psychiatry 22 (4), 519e526. http://doi.org/10.1038/mp.2016.146. Rodgers, A., Morgan, C., Bronson, S., Revello, S., Bale, T., 2013. Paternal stress exposure alters sperm microRNA content and reprograms offspring HPA stress axis regulation. Journal of Neuroscience 33 (21), 9003e9012. Rodgers, Morgan, C.P., Leu, N.A., Bale, T.L., 2015. Transgenerational epigenetic programming via sperm microRNA recapitulates effects of paternal stress. Proceedings of the National Academy of Sciences 112 (44), 13699e13704. http://doi.org/10.1073/pnas.1508347112. Roth, T.L., Lubin, F.D., Funk, A.J., Sweatt, J.D., 2009. Lasting epigenetic influence of early-life adversity on the BDNF gene. Biological Psychiatry 65 (9), 760e769. http://doi.org/10.1016/j.biopsych.2008.11.028. Ruden, D.M., Jamison, D.C., Zeeberg, B.R., Garfinkel, M.D., Weinstein, J.N., Rasouli, P., Lu, X., 2008. The EDGE hypothesis: epigenetically directed genetic errors in repeat-containing proteins (RCPs) involved in evolution, neuroendocrine signaling, and cancer. Frontiers in Neuroendocrinology. http://doi.org/10.1016/j.yfrne.2007.12.004. Saavedra-Rodríguez, L., Feig, L.A., 2013. Chronic social instability induces anxiety and defective social interactions across generations. Biological Psychiatry 73 (1), 44e53. http://doi.org/10.1016/j.biopsych.2012.06.035. Sagi-Schwartz, A., van IJzendoorn, M.H., Bakermans-Kranenburg, M.J., 2008. Does intergenerational transmission of trauma skip a generation? No meta-analytic evidence for tertiary traumatization with third generation of Holocaust survivors. Attachment and Human Development 10 (2), 105e121. http://doi.org/10.1080/ 14616730802113661. Sanabria, M., Cucielo, M.S., Guerra, M.T., dos Santos Borges, C., Banzato, T.P., Perobelli, J.E., et al., 2016. Sperm quality and fertility in rats after prenatal exposure to low doses of TCDD: a three-generation study. Reproductive Toxicology 65, 29e38. http://doi.org/10.1016/j.reprotox.2016.06.019. Savell, K.E., Gallus, N.V.N., Simon, R.C., Brown, J.A., Revanna, J.S., Osborn, M.K., et al., 2016. Extra-coding RNAs regulate neuronal DNA methylation dynamics. Nature Communications 7 (May), 12091. http://doi.org/10. 1038/ncomms12091. Scharf, M., Mayseless, O., 2011. Disorganizing experiences in second- and third-generation holocaust survivors. Qualitative Health Research 21 (11), 1539e1553. http://doi.org/10.1177/1049732310393747. Schmidt, E., Kornfeld, J.W., 2016. Decoding Lamarck???transgenerational control of metabolism by noncoding RNAs. Pfluegers Archiv European Journal of Physiology. http://doi.org/10.1007/s00424-016-1807-8. Sharma, 2014. Bioinformatic analysis revealing association of exosomal mRNAs and proteins in epigenetic inheritance. Journal of Theoretical Biology 357, 143e149. http://doi.org/10.1016/j.jtbi.2014.05.019. Sharma, A., 2013. Transgenerational epigenetic inheritance: focus on soma to germline information transfer. Progress in Biophysics and Molecular Biology 113 (3), 439e446. http://doi.org/10.1016/j.pbiomolbio.2012.12.003. Sharma, U., Conine, C.C., Shea, J.M., Boskovic, A., Derr, A.G., Bing, X.Y., et al., 2016. Biogenesis and function of tRNA fragments during sperm maturation and fertilization in mammals. Science 351 (6271), 391e396. http:// doi.org/10.1126/science.aad6780. Skinner, M.K., 2015. Environmental epigenetics and a unified theory of the molecular aspects of evolution: a neoLamarckian concept that facilitates neo-Darwinian evolution. Genome Biology and Evolution 7 (5), 1296e1302. http://doi.org/10.1093/gbe/evv073. Smith, Z.D., Chan, M.M., Mikkelsen, T.S., Gu, H., Gnirke, A., Regev, A., Meissner, A., 2012. A unique regulatory phase of DNA methylation in the early mammalian embryo. Nature 484 (7394), 339e344. http://doi.org/10. 1038/nature10960. Stilling, R.M., Dinan, T.G., Cryan, J.F., 2014. Microbial genes, brain & behaviour - epigenetic regulation of the gutbrain axis. Genes, Brain and Behavior 13 (1), 69e86. http://doi.org/10.1111/gbb.12109. Storm, J.J., Lima, S.L., 2010. Mothers forewarn offspring about predators: a transgenerational maternal effect on behavior. The American Naturalist 175 (3), 382e390. http://doi.org/10.1086/650443. Suh, N., Blelloch, R., 2011. Small RNAs in early mammalian development: from gametes to gastrulation. Development 138 (9), 1653e1661. http://doi.org/10.1242/dev.056234. Surani, A., 2016. Breaking the germ lineesoma barrier. Nature Reviews Molecular Cell Biology 17, 136. Todrank, J., Heth, G., Restrepo, D., 2011. Effects of in utero odorant exposure on neuroanatomical development of the olfactory bulb and odour preferences. Proceedings of the Royal Society B: Biological Sciences 278 (1714), 1949e1955. http://doi.org/10.1098/rspb.2010.2314. Tost, J., 2010. DNA methylation: an introduction to the biology and the disease-associated changes of a promising biomarker. Molecular Biotechnology. http://doi.org/10.1007/s12033-009-9216-2.

290

18. Stress and its effects across generations

Tweedie-Cullen, R.Y., Brunner, A.M., Grossmann, J., Mohanna, S., Sichau, D., Nanni, P., et al., 2012. Identification of combinatorial patterns of post-translational modifications on individual histones in the mouse brain. PLoS One 7 (5). http://doi.org/10.1371/journal.pone.0036980. van Steenwyk, G., Roszkowski, M., Manuella, F., Franklin, T., Mansuy, I.M., 2018. Transgenerational inheritance of behavioral and metabolic effects of paternal exposure to traumatic stress in early postnatal life: Evidence in the 4th generation. Environmental Epigenetics 4, 023. https://doi.org/10.1093/eep/dvy023. Vassoler, Byrnes, E.M., Pierce, R.C., 2014. The impact of exposure to addictive drugs on future generations: physiological and behavioral effects. Neuropharmacology. http://doi.org/10.1016/j.neuropharm.2013.06.016. Vassoler, White, S.L., Schmidt, H.D., Sadri-Vakili, G., Pierce, R.C., 2013. Epigenetic inheritance of a cocaine resistance phenotype. Nature Neuroscience 16 (1), 42e47. http://doi.org/10.1038/nn.3280. Watson, J.B., Mednick, S.A., Huttunen, M., Wang, X., 1999. Prenatal teratogens and the development of adult mental illness. Development and Psychopathology 11 (3), 457e466. http://doi.org/10532619. Weaver, I.C.G., Cervoni, N., Champagne, F. a, D’Alessio, A.C., Sharma, S., Seckl, J.R., et al., 2004. Epigenetic programming by maternal behavior. Nature Neuroscience 7 (8), 847e854. http://doi.org/10.1038/nn1276. Weinstock, M., 2008. The long-term behavioural consequences of prenatal stress. Neuroscience and Biobehavioral Reviews. http://doi.org/10.1016/j.neubiorev.2008.03.002. Weiss, I.C., Franklin, T.B., Vizi, S., Mansuy, I.M., 2011. Inheritable effect of unpredictable maternal separation on behavioral responses in mice. Frontiers in Behavioral Neuroscience 5 (February), 3. http://doi.org/10.3389/fnbeh.2011.00003. Wu, L., Lu, Y., Jiao, Y., Liu, B., Li, S., Li, Y., et al., 2016. Paternal psychological stress reprograms hepatic gluconeogenesis in offspring. Cell Metabolism 23 (4), 735e743. http://doi.org/10.1016/j.cmet.2016.01.014. Yao, Y., Robinson, A.M., Zucchi, F.C., Robbins, J.C., Babenko, O., Kovalchuk, O., et al., 2014. Ancestral exposure to stress epigenetically programs preterm birth risk and adverse maternal and newborn outcomes. BMC Medicine 12 (1), 121. http://doi.org/10.1186/s12916-014-0121-6. Yehuda, R., Bierer, L.M., Andrew, R., Schmeidler, J., Seckl, J.R., 2009. Enduring effects of severe developmental adversity, including nutritional deprivation, on cortisol metabolism in aging Holocaust survivors. Journal of Psychiatric Research 43 (9), 877e883. http://doi.org/10.1016/j.jpsychires.2008.12.003. Yehuda, R., Engel, S.M., Brand, S.R., Seckl, J., Marcus, S.M., Berkowitz, G.S., 2005. Transgenerational effects of posttraumatic stress disorder in babies of mothers exposed to the World Trade Center attacks during pregnancy. Journal of Clinical Endocrinology and Metabolism 90 (7), 4115e4118. http://doi.org/10.1210/jc.2005-0550. Yehuda, R., Halligan, S.L., Bierer, L.M., 2001. Relationship of parental trauma exposure and PTSD to PTSD, depressive and anxiety disorders in offspring. Journal of Psychiatric Research 35 (5), 261e270. http://doi.org/10.1016/ S0022-3956(01)00032-2. Yehuda, R., Teicher, M.H., Seckl, J.R., Grossman, R.A., Morris, A., Bierer, L.M., 2007. Parental posttraumatic stress disorder as a vulnerability factor for low cortisol trait in offspring of holocaust survivors. Archives of General Psychiatry 64 (9), 1040e1048. http://doi.org/10.1001/archpsyc.64.9.1040. Yohn, N.L., Bartolomei, M.S., Blendy, J.A., 2015. Multigenerational and transgenerational inheritance of drug exposure: the effects of alcohol, opiates, cocaine, marijuana, and nicotine. Progress in Biophysics and Molecular Biology. http://doi.org/10.1016/j.pbiomolbio.2015.03.002. Younger, S.T., Corey, D.R., 2011. Transcriptional gene silencing in mammalian cells by miRNA mimics that target gene promoters. Nucleic Acids Research 39 (13), 5682e5691. http://doi.org/10.1093/nar/gkr155. Zannas, A.S., West, A.E., 2014. Epigenetics and the regulation of stress vulnerability and resilience. Neuroscience. http://doi.org/10.1016/j.neuroscience.2013.12.003. Zerach, G., Solomon, Z., 2016. Low levels of posttraumatic stress symptoms and psychiatric symptomatology among third-generation Holocaust survivors whose fathers were war veterans. Journal of Psychiatric Research 73, 25e33. http://doi.org/10.1016/j.jpsychires.2015.11.014. Zhang, H., Niu, B., Hu, J.F., Ge, S., Wang, H., Li, T., et al., 2011. Interruption of intrachromosomal looping by CCCTC binding factor decoy proteins abrogates genomic imprinting of human insulin-like growth factor II. Journal of Cell Biology 193 (3), 475e487. http://doi.org/10.1083/jcb.201101021. Zhou, Q., Nie, R., Prins, G.S., Saunders, P.T.K., Katzenellenbogen, B.S., Hess, R. a., 2002. Localization of androgen and estrogen receptors in adult male mouse reproductive tract. Journal of Andrology 23 (6), 870e881. http:// doi.org/10.1002/j.1939-4640.2002.tb02345.x.

C H A P T E R

19

Corticolimbic stress regulatory circuits, hypothalamoepituitarye adrenocortical adaptation, and resilience James P. Herman Department of Pharmacology and System Physiology, University of Cincinnati, Cincinnati, OH, United States

Glucocorticoid signaling, stress, and reslience Glucocorticoid signaling plays a major role in both chronic stress adaptation and chronic stress pathologies. These hormones are vital components of stress responses and are absolutely critical for successful coping with environmental or physiological adversity. On the other hand, prolonged perturbations in glucocorticoid homeostasis are linked to the development of stress-related affective conditions and exacerbation of systemic pathologies (Gold and Chrousos, 1999; Munck et al., 1984; Yehuda, 2002). Consequently “dialing in” the glucocorticoid response to match demand is a key component of stress adaptation and resilience. This is accomplished in large part by glucocorticoid negative feedback signaling, whereby the hormone binds to cognate receptors (mainly glucocorticoid receptors or GRs) to inhibit further activation of the neural and pituitary arms of the hypothalamoepituitarye adrenocortical (HPA) axis (hypothalamic paraventricular nucleus (PVN), corticotropinreleasing hormone (CRH) neurons, and adenohypophysial corticotropes, respectively) (Keller-Wood and Dallman, 1984; Myers et al., 2012). Feedback is a complex process that involves direct inhibition of effector CRH neurons (Tasker and Herman, 2011) and pituitary adrenocorticotrophic hormone (ACTH) release (John et al., 2004); signaling via multiple extrahypothalamic brain regions (see below) (Myers et al., 2012); as well as that emanating from other body compartments (e.g., adipocytes) (de Kloet and Herman, 2018). This distributed feedback process offers multiple sites for adjustment of HPA output and the ability to fine-tune

Stress Resilience https://doi.org/10.1016/B978-0-12-813983-7.00019-7

291

Copyright © 2020 Elsevier Inc. All rights reserved.

292

19. Corticolimbic stress regulatory circuits, hypothalamoepituitaryeadrenocortical adaptation, and resilience

HPA output to cope with psychological or physiological/metabolic demand. However, distributed feedback has a downside, wherein disruption of checks and balances can occur at any number of places in the brain or body. Loss of appropriate signal integration may underlie breakdowns in capacity for resilience and adaptation, allowing pathological actions of glucocorticoids and more importantly glucocorticoid-regulated systems to become manifest. When considering stress resilience from the perspective of the HPA axis, it is important to consider that glucocorticoids play different roles in acute versus chronic time domains. The acute corticosteroid response is almost certainly adaptive, whereas prolonged responses are more complicated (de Kloet et al., 2005; Munck et al., 1984). It is clear that even within the context of chronic stress the corticosteroid response is needed to cope with individual adverse situations and is engaged in and is part of the adaptive process. It is likely that situation-appropriate HPA axis drive contributes to resilience in this regard, and thus enhanced response capacity and sensitized responses may be beneficial to the organism in times of anticipated need. At the whole organism level, the “adaptive” chronic stress response is evident in most experimental regimens, in which animals exhibit an ability to maintain health and viability even during very long exposure periods. One is hard pressed to find any reports of significant physical morbidity or mortality following most standard stress regimens, e.g., 4 weeks of chronic restraint (Chiba et al., 2012), 4 weeks of chronic variable stress (Herman et al., 1995), and several weeks of chronic mild stress (Willner, 2017). Indeed, in our group, we have experienced only 2 deaths associated with chronic variable stress in some 20 years, those two incidences being geriatric animals that may have had existing complicating pathologies. Thus, one can conclude that the physiological and behavioral consequences of chronic stress occur within the context of physiological adaptation. This is not to say that pathology is not a feature of chronic stress, as stress regimens featuring high intensity stressors (severe chronic social instability regimen) can cause lifethreatening adrenal insufficiency and significant morbidity (e.g., susceptibility to severe gastric ulceration) (Langgartner et al., 2015, 2017). Hence, what is measured in most chronic stress regimens is largely characteristic of a context-appropriate adaptive response, i.e., some degree of physical resilience. “Maladaptative” chronic stress responses occur when the magnitude of the response does not match the needs appropriate to coping with acute or aggregate stressor exposure. In terms of the HPA axis, this would amount to either glucocorticoid hyper- or hyposecretion. Either may be construed as maladaptive, as too little response may not sufficiently engage needed systems, whereas overshoot could promote catabolic glucocorticoid actions (De Kloet et al., 1998). Indeed, diseases of adaptation (e.g., posttraumatic stress disorder, depression, etc) can be associated with either over- or underactivation of the HPA axis (Gold and Chrousos, 1999; Yehuda, 2002). Thus, the failure of resilience in the face of chronic stress has to be measured with respect to the normal adaptation of the system. This means that chronic stress-induced changes in the HPA axis should be considered “normal,” and deviations from the new response characteristics “maladaptive.” Thus treatments that reduce the HPA axis response to chronic stress do not necessarily indicate resilience, as these are potentially as bad as or worse than the treatments that cause glucocorticoid excess. In the remainder of this review, we will focus on control of the “chronic stress resilience” phenotype from the perspective of neural mechanisms of HPA axis control. Although the concept of resilience spans many systems, the focus of this discussion will be directed at

Limbic regulation of hypothalamoepituitaryeadrenocortical axis stress responses

293

how perturbations of corticolimbic stress regulatory nodes alter the HPA axis “resilience” phenotype and circuit mechanisms that may underlie the ultimate impact of these regions on the HPA axis. It is important to consider that the HPA axis is but one stress adaptation mechanism, and indeed, it is likely that others operate along the same principles. For example, processes that mediate behavioral adaption and resilience require a similar dynamic: an initial response that evokes profound reactions (e.g., freezing after an intense fearful experience) and habituation or extinction of the initial response to allow for behavioral flexibility in the future. Thus, while the remainder of this chapter will concentrate on HPA axis adaptation and resilience, it is important to keep in mind that other stress response pathways likely follow similar rules.

Neural circuitry of stress regulation Much of the understanding of stress control circuitry has grown from work mapping brain regions responsible for activation and inhibition of the HPA axis. This initial emphasis was likely based on the relative ease in recording the “readout” of the HPA axis, glucocorticoid secretion. In the most simple case, activation of the HPA axis by a stressor consists of a more or less time-locked secretory episode, followed by a more protracted return to baseline (Herman et al., 2016). The dynamics of the HPA response allows one to lesion, simulate, or pharmacologically modulate specific brain regions and subsequently record impact on response magnitude using very simple blood-sampling procedures and small volumes of blood. The ease of readout detection is not shared by other stress effector pathways, e.g., the autonomic nervous system, and requires sophisticated and often unwieldy recording apparati for assessment of heart rate and/or blood pressure and/or laborious sampling procedures and relatively insensitive assays for accurate analysis of plasma catecholamines. Circuit analyses for behavioral end points have proven more difficult to align onto “stress pathways,” as they are often mapped onto different constructs (e.g., “anxiety,” “depression,” and even “addiction”). Although the ability to monitor autonomic and behavioral stress readouts is improving rapidly (e.g., cardiotelemetry allows comprehensive recording of cardiovascular stress responses before, during, and after stress exposure, optochemogenetic approaches can resolve connectional involvement of defined cellular phenotypes), much of our current knowledge base on stress circuitry is derived from what we know about the organization of HPA axis inputs. It is important to consider that control of autonomic and behavioral stress responses may follow the same “tracks,” albeit with parallel outputs (see (Ulrich-Lai and Herman, 2009)).

Limbic regulation of hypothalamoepituitaryeadrenocortical axis stress responses: hippocampus, amygdala, and prefrontal cortex General organizational scheme of limbic stress regulation In general, regulation of psychogenic stress responses is controlled in top-down fashion by integrated activation of the ventral hippocampus, several amygdala subnuclei, and

294

19. Corticolimbic stress regulatory circuits, hypothalamoepituitaryeadrenocortical adaptation, and resilience

FIGURE 19.1 Organization of limbic stress relays. Limbic modulation of stress responses occurs predominantly via oligosynaptic inputs to the paraventricular nucleus (PVN) of the hypothalamus and other preautonomic brain regions. Excitatory inputs are colored blue with solid lines and inhibitory inputs (GABA) are colored red with dashed lines. Top: The ventral subiculum (vSUB) coordinates hippocampal stress output by providing glutamatergic input to primarily inhibitory PVN relays, thereby limiting psychogenic stress responses. Middle: GABAergic projections from the central amygdala (CeA) regulate responses to systemic stressors, whereas those from the medial amygdala (MeA) preferentially modulate responses to psychogenic stressors. Through glutamatergic projections within and outside the amygdala, the basolateral amygdala (BLA) plays a role in both the acute response to psychogenic stress and in chronic stress regulation. Bottom: The prelimbic cortex (PL) inhibits responses to psychogenic stress, and this inhibition is mediated predominantly by glutamatergic projections to inhibitory PVN relays. In contrast, the infralimbic cortex (IL) activates autonomic and possibly hypothalamoepituitaryeadrenocortical axis responses to psychogenic stress, perhaps via direct (nucleus of the solitary tract, NTS) or indirect (CeA) projections. anteriomedial BST (amBST), anteroventral BST (avBST), bed nucleus of the stria terminalis (BST), dorsal raphe nucleus (DRN), dorsomedial hypothalamus (DMH), lateral septum (LS), medial preoptic area (mPOA), paraventricular thalamus (PVT), peri-PVN (pPVN), posteromedial BST (pmBST), ventral subiculum (vSub). Modified from Ulrich-Lai, Y.M., Herman, J.P. (2009). Neural regulation of endocrine and autonomic stress responses. Nature Reviews Neuroscience 10 (6), 397e409 with permission.

ventromedial regions of the prefrontal cortex (vmPFC) (see Ulrich-Lai and Herman, 2009). These regions have little direct input into the PVN, relying on intermediary synaptic relays to convey information to PVN CRH neurons (Ulrich-Lai and Herman, 2009) (Fig. 19.1). Previous studies indicate that the majority of defined limbic-PVN relay neurons (e.g., in the bed nucleus of the stria terminalis, dorsomedial hypothalamus, preoptic area) are GABAergic, presenting an opportunity for modulation of HPA activity by adjusting PVN inhibitory tone. Of note, hippocampal and prefrontal cortical projection neurons are glutamatergic, whereas amygdala output is heavily GABAergic (see Ulrich-Lai and Herman, 2009). Consequently, hippocampal and cortical inputs are likely to excite PVN-projecting neurons and effect response inhibition, whereas amygdala neurons are likely to excite PVN neurons by transsynaptic disinhibition (Herman and Cullinan, 1997; Ulrich-Lai and Herman, 2009) (Fig. 19.1). The majority of limbic-PVN relays are found in brain regions that are also important for homeostatic balance, in terms of regulating autonomic function and metabolism. As these PVN-projecting regions receive input from peripheral homeostats as well as limbic sites,

Limbic regulation of hypothalamoepituitaryeadrenocortical axis stress responses

295

they are positioned to integrate the “perceived” needs of the organism (limbic) with needs engendered by physiological demands (Herman and Cullinan, 1997). This allows for “competing” interests and under normal conditions may allow homeostatic challenge to attenuate limbic input and thereby promote the appropriate survival response. Conversely, pathological alterations of limbic input can place pressure on homeostatic pathways and may be a factor in the stunning comorbidity between mood and metabolic symptoms in diseases of adaptation.

Hippocampus Evidence for a role of the hippocampus in stress inhibition is strong, translating across rodent models, nonhuman primates, as well as humans (Herman et al., 2003). Drive of hippocampal output neurons inhibits HPA axis responses to stress, whereas lesions enhance reactivity to psychogenic (but not systemic) stressors (Herman et al., 1998). This distinction is important, as it speaks to the differential importance of the hippocampus (as well as other corticolimbic structures, below) in processing multimodal sensory information as opposed to homeostatic signals. Lesions studies relegate primary stress regulatory processing to the ventral region of the hippocampus. Indeed, hippocampal connections to the ventral subiculum (SUBv) are particularly important, as selective damage to the latter region enhances CRH mRNA and peptide expression, stress-induced corticosterone secretion, and postrestraint Fos activation of PVN neurons (Herman et al., 2003; Radley and Sawchenko, 2011). Damage to the dorsal hippocampus/subiculum does not affect stress responses (but may blunt circadian corticosterone secretion, a topic beyond the scope of this Chapter) (Herman and Mueller, 2006). Importantly, damage to the SUBv does not affect basal HPA activation or responses to systemic stressors, suggesting importance is limited to control of psychogenic stress responses and does not affect ongoing glucocorticoid secretory patterns (Herman and Mueller, 2006). These data indicate active cognitive processing is required to reveal the role of SUBv in stress inhibition. The hippocampus has been touted as a putative feedback site for glucocorticoid inhibition of the HPA axis, as lesions are known to promote HPA axis stress responses, and local implants of glucocorticoids inhibit corticosterone release (although the evidence here is limited) (see Jacobson and Sapolsky, 1991 for review). Corticolimbic deletion of GR using a CaMKII promoter, which is highly expressed in the hippocampus, causes HPA axis hypersecretion to psychogenic (but not systemic) stressors and resistance to dexamethasone negative feedback in mice (Boyle et al., 2005; Furay et al., 2008). However, GR is also deleted in other HPA axiserelevant forebrain regions, such as the prefrontal cortex (PFC) and amygdala (see below), making it difficult to ascribe knockout effects to the hippocampus proper (Boyle et al., 2005; Furay et al., 2008). By and large, studies of the impact of hippocampal/SUBv on stress integration have focused largely on acute stress reactivity. However, several years ago, our group performed experiments focusing on chronic stress regulation in rats with SUBv lesions. As these studies have become relevant to the understanding of hippocampal regulation of chronic stress processing, we present the data here. Male SpragueeDawley rats received intra-SUBv infusions of ibotenic acid or saline. Animals were allowed to recover for 21 days and were subsequently exposed to a 28-day chronic variable stress regimen or served as handled

296

19. Corticolimbic stress regulatory circuits, hypothalamoepituitaryeadrenocortical adaptation, and resilience

TABLE 19.1

Effects of ventral subiculum lesion on responses to chronic variable stress. Control Handled

Control CVS

Ventral subiculum (SUBv) lesion handled

SUBv lesion CVS

135 þ 8

74 þ 18*

150 þ 13

116 þ 9*#

b

Adrenal weight (mg/100g)

11.1 þ 0.7

14.1 þ 1.5*

11.1 þ 1.1

14.3 þ 0.7*

c

Thymus weight (mg/100g)

92.5 þ 5.2

69.4 þ 11.6*

93.4 þ 8.2

69.5 þ 11.6*

d

PVN CRH mRNA (corrected gray level)

73.8 þ 1.4

86.5 þ 2.3^

80.7 þ 3

87.6 þ 4.4

e

Open arm time

0.5 þ 0.4

0.2 þ 0.9

2.6 þ 0.6#

1.1 þ 0.3#

f

Grooming

3.5 þ 0.5

0.1 þ 1.1*

3.0 þ 0.8

3.9 þ 0.5^

0.8 þ 0.3

2.5 þ 0.7*

0.8 þ 0.5

1.2 þ 0.4$

a

Body weight change

g

Rearing

CRH, corticotropin-releasing hormone; CVS, chronic variable stress; PVN, paraventricular nucleus; a Lesion: F(1,24) ¼ 5.11, P < .05; stress: F(1, 24) ¼ 14.3, P control, P < .05, #, SUBv lesion > control, P < .05. b Stress: F(1,24) ¼ 8.35, P Control, P < .05. c Stress: F(1,24) ¼ 5.82, P < .05; *, CVS < Control, P < .05. d Lesion by stress interaction: F(1,24) ¼ 8.67, P < .05;^P < .05, control handled versus control CVS. e Lesion: F(1,24) ¼ 5.74, P < .05; #, lesion > control, P < .05. f Lesion: F(1,24) ¼ 4.70, P < .05; *, stress < control, P < .05;^, SUBv lesion CVS>control CVS. g Lesion: F(1,24) ¼ 4.70, P < .05; lesion  stress: F (1,24) ¼ 7.88, P < .05); *, CVS > control, P < .05P < .05, $, SUBv lesion CVS < control CVS.

controls (see Herman et al., 1995). Animals were exposed to an elevated plus maze (5 min in a standard elevated plus maze (EPM) under standard illumination) as the morning stressor on day 24. Following EPM, blood samples were collected by tail nick 30, 60, and 120 min following stress initiation. Animals were killed by rapid decapitation on day 29, the morning after the final stressor. All procedures were approved by the institutional IACUC. Body weights, adrenal weights, and thymus weights are reported in Table 19.1. CRH mRNA was assessed using in situ hybridization histochemistry, according to previously published methods. Behavior in the EPM was videotaped and open arm time, grooms, and rears quantified. Corticosterone was determined by standard radioimmunoassay (see Herman et al., 1995). Lesions do not appear to alter chronic variable stress-induced adrenal hypertrophy, thymic atrophy, or weight reduction, and chronic stress does not increase PVN CRH mRNA expression beyond that of lesion alone (see Table 19.1). There was no effect of lesion or chronic variable stress (CVS) on resting corticosterone and no effect of either manipulation on the corticosterone response to EPM exposure. SUBv lesion increased open arm time and blocks chronic stresseinduced decreases in rearing in the elevated plus maze (Table 19.1), suggesting that, if anything, loss of SUBv input attenuates the impact of chronic stress, at least in terms of anxiety-related behaviors. Thus, the effects of SUBv lesions on chronic stresserelated behavior do not recapitule or add to acute stress effects on the HPA axis, and if anything may alter behavioral adaptation.

Amygdala In general, the weight of data prescribes a role for amygdalar nuclei in stress (and HPA axis) excitation. Principal stress-related output neurons of the amygdala are localized in

Limbic regulation of hypothalamoepituitaryeadrenocortical axis stress responses

297

medial (MeA) and central nuclei of the amygdala (CeA) and are predominantly GABAergic (Herman et al., 2003; Ulrich-Lai and Herman, 2009). In general, there is little evidence for substantial direct innervation of the PVN by CeA and MeA neurons (Canteras et al., 1995; Prewitt and Herman, 1998), suggesting the need for intermediary relay neurons. The MeA appears to be involved in driving HPA axis responses, particularly in response to psychogenic stressors. The MeA is selectively activated by novelty, social defeat, restraint, and predator exposure but not by “systemic” stimuli such as ether vapors or immune stimuli (Dayas et al., 2001; Emmert and Herman, 1999; Figueiredo et al., 2003; Kollack-Walker and Newman, 1995). Lesions of the MeA attenuate HPA axis responses to restraint, predator odor, and sensory stimuli but not to interleukin 1 beta or ether (Dayas et al., 1999; Feldman et al., 1994; Solomon et al., 2010). Conversely, stimulation of the MeA region increases corticosterone release in unstressed animals, effects that are blocked by lesions of primary MeA outflow in the stria terminalis, or putative relay targets in the bed nucleus of the stria terminalis (BST) and preoptic area (Feldman et al., 1990). At least part of the excitatory effect of the MeA may be mediated by central melanocortin signaling, as intra-MeA infusion of a MC4R agonist drives HPA axis and emotional responses, whereas antagonist infusions block anxiogenic, anorectic, and HPA axis responses to restraint stress(Liu et al., 2013). Despite the ability of the MeA to promote psychogenic stress responses, this region does not appear to be involved in driving chronic stress activation of the HPA axis in any simple way. The MeA does not show exaggerated activation (FosB staining) following chronic stress (Flak et al., 2012), and lesions of the MeA do not affect HPA axis responses to chronic variable stress exposure (although they do attenuate chronic stresseinduced weight loss) (Solomon et al., 2010). Thus, as was the case with the SUBv, effects of lesions on acute stress reactivity are not extended into the chronic time frame. In contrast to the MeA, activation of the CeA occurs primarily in response to systemic stressors, such as interleukin 1 beta, hypoxia, and hemorrhage (Dayas et al., 2001; Figueiredo et al., 2003; Thrivikraman et al., 1997), but not following swim, noise, or restraint (Cullinan et al., 1996; Dayas et al., 2001). The CeA also appears reactive to painful stimuli, including acute visceral pain (Traub et al., 1996), as well as predator scent (Day et al., 2004). Lesion studies are in general agreement, wherein damage to the CeA disrupts HPA axis responses to immune challenge but not to restraint (Prewitt and Herman, 1997; Thrivikraman et al., 1997). However, it should be noted that CeA damage attenuates acute bradycardia seen following social stress (Roozendaal et al., 1990), suggesting that this region may differentially affect autonomic versus neuroendocrine effector systems. It is important to also note that the CeA expresses abundant levels of CRH mRNA and peptide and that CRH expression is potentiated by footshock (Makino et al., 1999), predator exposure (Figueiredo et al., 2003), visceral pain (Kim et al., 2010), and elevated levels of glucocorticoids (Makino et al., 1994). In sheep, acute stress (dog) causes intraamygdalar release of CRH, and detectable CRH to a novel stressor is enhanced after chronic stress (dog) exposure (Cook, 2002). These studies have been interpreted to suggest that upregulation of CRH plays a role in sensitization of HPA axis stress responses following chronic stress. However, lesions of the CeA do not affect HPA axis drive or PVN CRH expression following chronic variable stress exposure (Prewitt and Herman, 1997), arguing against a causal role of the CeA, at least in HPA axis drive.

298

19. Corticolimbic stress regulatory circuits, hypothalamoepituitaryeadrenocortical adaptation, and resilience

The basolateral amygdala (BLA) is thought to be upstream of the MeA and CeA and may be essential for coordination of amygdala output. This region shows limited Fos activation by psychogenic stimuli (Cullinan et al., 1995; Emmert and Herman, 1999), suggesting a possible role in drive of HPA axis output. There are some data to suggest that BLA lesions can attenuate HPA axis responses to restraint (Bhatnagar et al., 2004). However, reduced HPA axis responsiveness is not observed in all studies (see Ulrich-Lai et al., 2010) and is not recapitulated by local pharmacological inhibition of the BLA (Bhatnagar et al., 2004). However, unlike the MeA and CeA, the BLA appears to be required for habituation of responses to repeated stress, suggesting a role in resilience. For example, BLA lesions or local inhibition of beta-adrenergic signaling blocks reductions in HPA axis reactivity normally seen following repeated exposure to restraint stress (Bhatnagar et al., 2004; Grissom and Bhatnagar, 2011). Moreover, fos mRNA mapping and lesion studies suggest that the BLA is responsible for stress-buffering effects of chronic reward on the HPA axis (Ulrich-Lai et al., 2010), suggesting a role for this region in conferring stress resilience.

Medial prefrontal cortex The vmPFC plays a complex (if not confusing) role in regulation of stress responses. The vmPFC can be roughly divided into the ventralmost infralimbic cortex (IL) and the overlying prelimbic cortex (PL), roughly defined on the basis of connectivity (Ongur and Price, 2000). The IL is heavily connected with autonomic and neuroendocrine efferent circuits (e.g., nucleus of the solitary tract [NTS]) and is thought to provide “visceromotor” drive during emotional responses (Ongur and Price, 2000). The PL projects heavily to regions involved in reward (e.g., nucleus accumbens) and is implicated in reinforcement processing (Vertes, 2004). While distinct outputs can be defined for dorsal and ventralmost cell groups, the two regions heavily interact with one another and have a number of overlapping projections to stress and reward sites. Importantly, despite proximity and connectional overlap, the involvement of IL and PL in emotional processing can differ substantially. For example, lesion and stimulation/inhibition studies suggest that the PL is required for expression of conditioned fear, whereas the IL is critical for recall of extinction learning (Vidal-Gonzalez et al., 2006). Numerous studies have assessed the role of the vmPFC in HPA axis regulation. Lesions that encompass both the PL and IL typically result in increased HPA axis responses to acute psychogenic (but not systemic) stressors (Diorio et al., 1993; Figueiredo et al., 2003; Radley et al., 2006). Effects on stress inhibition are recapitulated by more refined lesions of the PL, and indeed, stimulation of the PL is sufficient to inhibit HPA axis stress responses and PVN Fos activation (Jones et al., 2011; Radley et al., 2006), suggesting this subregion plays a critical role in controlling hormonal reactivity to acute stress. Establishing the role of the IL has proven more elusive. Large lesions that encompass the IL and ventral PL (collectively, the ventromedial PFC vmPFC) decrease resting corticosterone secretion and dampen responses to repeated restraint exposure, suggesting a role in enhancing stress adaptation (Sullivan and Gratton, 1999; Radley et al., 2006). Remarkably, these effects are recapitulated by lesions compromising only the right vmPFC, indicating lateralization of stress regulatory output (Sullivan and Gratton, 1999). Unlike the PL, excitotoxic lesions largely confined to the IL do not affect HPA axis responses to acute stress (Radley et al., 2006).

Integration of hippocampal, prefrontal, and amygdala projections

299

However, recent studies using viral vectors to deplete IL vesicular glutamate packaging causes exaggerated responses to acute restraint (Myers et al., 2017), suggesting that glutamatergic outflow from the IL is a key modulator of HPA axis inhibition by the downstream relays. The IL and PL are targets for glucocorticoid signaling. Both regions express abundant levels of both GR and mineralocorticoid receptor (MR), and sizable populations of GR immunoreactive neurons express Fos following restraint stress (Ostrander et al., 2003), suggesting that glucocorticoids may be important modulators of stress-activated pathways, in accordance with the IL/PL being a potential feedback integrative node. Accordingly, local implants of glucocorticoids into the IL/PL inhibit corticosterone responses to acute psychogenic stress (Diorio et al., 1993). In addition, local viral knockdown of GR in either IL or PL neurons increase ACTH and corticosterone responses to restraint, consistent with a role for local glucocorticoid feedback in modifying stress reactivity (McKlveen et al., 2013). Glucocorticoids are known to inhibit interneurons (e.g., increase miniature inhibitory postsynaptic currents [mIPSCs]) in the PL (Hill et al., 2011), and this may be important in gating glutamate outflow to subcortical stress relays sites. Chronic stress causes marked long-term activation of neurons (as determined from FosB aggregation) in both the PL and IL (Flak et al., 2012). Indeed, significant FosB activation is not observed in other limbic stress regulation nodes (hippocampus or amygdala subnuclei) (Flak et al., 2012), highlighting the potential importance of the vmPFC region in reading stressor chronicity. It is notable that increases in FosB are observed following exposure to a chronic variable stress regimen but not to repeated restraint, connecting FosB induction to long-term exposure to uncertain and unpredictable outcomes (Flak et al., 2012). The IL appears to be critical for regulating responses to chronic stress, as viral knockdown of GR in the IL (but not PL) sensitizes HPA axis reactivity following chronic stress (McKlveen et al., 2013). Chronic stress hypersensitivity is also observed following reduction of IL glutamate packaging (Myers et al., 2017). Mechanistically, chronic stress may affect HPA axis signaling via reduction of stress-inhibitory IL outflow, mediated by increased innervation of IL projection neurons by GABAergic interneurons (McKlveen et al., 2016). In addition, recent work indicates that damage to the IL (but not PL) blocks the ability of environmental enrichment to inhibit behavioral and physiological consequences of chronic stress, implying a role for the IL in conferring resilience. Overall, the respective roles of the PL and IL appear to be dependent on both the nature of the stressor and its chronicity. Given its importance in complex cognitive processing, it is perhaps not surprising to learn that the output gated by the PFC depends on the context of the situation at hand. In particular, the IL appears to be very much tuned to stressor chronicity, as the clearest effects of IL manipulations occur in the context of chronic stress (Fig. 19.2). The output of the IL is sensitive to the stress hormone environment and may be modulated under conditions where continued drive may be important, e.g., when the “world is a hostile place.”

Integration of hippocampal, prefrontal, and amygdala projections The SUBv, amygdala, and vmPFC have overlapping as well as distinct projections to a variety of subcortical sites that ultimately innervate the PVN (as well as other subcortical

300

19. Corticolimbic stress regulatory circuits, hypothalamoepituitaryeadrenocortical adaptation, and resilience

FIGURE 19.2 Neural mechanisms controlling chronic stress regulation of the hypothalamoepituitarye adrenocortical (HPA) axis. Pathways responsible for drive of the HPA axis under chronic stress are not as well understood as those mediating acute response. There is strong evidence that the paraventricular thalamus (PVT), which is not involved in acute stress excitation or inhibition, is required for both stress habituation and stress facilitation, suggesting a role in communicating stress chronicity. Importantly, the PVT has extensive reciprocal projections to the infralimbic (IL) cortex, prelimbic (PL) cortex, and ventral subiculum, as well as projections to the area of the BST. Neuronal activation studies indicate the existence of a small network of structures that are differentially activated by chronic unpredictable stress (relative to restraint), including the IL, PL, posterior hypothalamus (PH), and nucleus of the solitary tract (NTS). Importantly, the PH and NTS are both connected with the IL and both mediate acute stress excitation, suggesting a possible integrated circuit mediating chronic stress drive. Finally, chronic stress increases tone of corticotropin-releasing hormone (CRH)eexpressing stress circuitry, suggesting that CRH systems (e.g., central amygdala [CeA]) may be recruited by chronic stress and participate in HPA axis hyperdrive. Adapted from Herman, J.P., Ostrander, M.M., Mueller, N.K. and Figueiredo, H (2005). Limbic system mechanisms of stress regulation: hypothalamopituitary-adrenocortical axis. Prog Neuropsychopharmacol Biol Psychiatry 29, 1201e1213.

structures regulating autonomic and behavioral stress responses) (Fig. 19.1). Areas of convergence may be critical for integration of diverse information coming from the various corticolimbic regions. Recent studies indicate that convergence may very well occur at the level of individual neurons located in relay sites. Anatomical studies indicate that terminals from SUBv and PL neurons converge on the same neuron in the anteroventral BST (Radley and Sawchenko, 2011). Convergent information from the SUBv and PL appears to be functionally significant, as dual lesion studies indicate that the combined impact of damage to both structures is additive (Radley and Sawchenko, 2011). Unpublished data from our group have noted similar convergence of inputs from MeA and vSUBv onto neurons in the posterior BST.

Integration of hippocampal, prefrontal, and amygdala projections

301

Bed nucleus of the stria terminalis As noted above, the BST is a site of convergence for descending corticolimbic outflow. The BST sends direct innervation to the PVN (Cullinan et al., 1993) and also projects to downstream regions involved in cardiovascular regulation, as well as defensive behaviors and fear responses (e.g., periaqueductal gray) (Dong and Swanson, 2004, 2006a,c). The ability to innervate multiple stress effectors suggests that the BST is involved in general integration of neuroendocrine, autonomic nervous system, and behavioral stress responses. The BST is anatomically complex. Consequently, it is perhaps not too surprising to find a microorganization of responses across different regions of the BST. Damage (excitoxic lesion) to the posterior BST enhances ACTH and corticosterone release following restraint, suggesting a role in mediating HPA axis responses to psychogenic stressors (Choi et al., 2007). This region is targeted by both the SUBv and IL/PL, as well as the MeA (Canteras et al., 1995; Cullinan et al., 1993; Vertes, 2004). Tracing studies indicate the existence of a direct relay between the SUBv and PVN in the BST (Cullinan et al., 1993). PVN projections from the posterior BST are GABAergic (Cullinan et al., 1993), consistent with transsynaptic inhibition. In contrast, lesions of the anterior BST inhibit HPA axis responses to acute psychogenic stress, suggesting the existence of an excitatory relay in this region (Choi et al., 2007). Of note, the ventral component of this region houses CRH as well as GABAergic neurons that project to the PVN (Dong and Swanson, 2004), of which the former may potentially be involved in stress activation. The role of the BST in chronic stress appears more complex. Lesions of the posterior BST increase responsiveness to novel stressors (Choi et al., 2008b), suggesting a role for this region in limiting HPA axis sensitization by chronic stress. One the other hand, enhanced chronic stress sensitization is also seen following anterior BST lesions (Choi et al., 2008a). These data indicate that the valence of anterior BST action varies with the chronicity of stress exposure. This implies a functional switch in anterior BST interaction with the PVN. It is possible that chronic stress may bias toward inhibitory output of the anterior BST, perhaps mediated by GABAergic projections, at the expense of excitatory CRH (or perhaps glutamatergic) drive. The mechanism for the “switch” in connection valence may involve differential engagement of corticolimbic outputs targeting individual subpopulations of BST neurons. In support of this hypothesis, animals exposed to a novel acute stress after chronic variable stress exposure show reduced Fos induction in PVN-projecting anterior BST neurons (Radley and Sawchenko, 2015), perhaps linked to decreased GABAeric inhibition (Fig. 19.2).

Paraventricular thalamus The paraventricular thalamus (PVT) has been implicated in processing of stressor chronicity (Fig. 19.2). Damage to the posterior PVT blocks chronic stresseinduced sensitization of the HPA axis (Bhatnagar and Dallman, 1998) and also inhibits habituation of HPA axis responses to repeated mild stress (e.g., restraint) (Bhatnagar et al., 2002). Importantly, the posterior PVT receives input from the SUBv, IL, PL, BLA, MeA, and CeA (Canteras et al., 1995; Cullinan et al., 1993; Li and Kirouac, 2012; Vertes, 2004), making this region a potential stress integration site. In addition, the posterior PVT also sends projections to both anterior and posterior divisions of the BST (Vertes and Hoover, 2008), an arrangement that may be of relevance to alteration of BST-PVN signaling in the face of chronic stress. It is also

302

19. Corticolimbic stress regulatory circuits, hypothalamoepituitaryeadrenocortical adaptation, and resilience

important to note that strong reciprocal projections from the posterior PVT to all of the regions above that provide input (Vertes and Hoover, 2008). Importantly, local blockade of glucocorticoid receptors (MR and GR) blocks habituation of HPA axis stress responses (Jaferi et al., 2003). These data suggest that as was the case for upstream corticolimbic stress regulatory region, glucocorticoids are essential in the computation process involved in driving adaptation and resilience (habituation of perhaps unneeded responses) in the face of chronic stress predictability.

Hypothalamic and brain stem circuitry A few years ago, we performed studies assessing FosB/deltaFosB expression in the brain, following either chronic variable stress or a habituating stress regimen (repeated restraint) (Flak et al., 2012). Our data suggested the existence of circuits that are selectively activated by unpredictable and nonhabituating stressor chronicity. As noted above, both IL and PL were included in this group. Importantly, selective engagement was also observed in two regions downstream of the IL (to a lesser extent PL), including the posterior hypothalamus and NTS (Flak et al., 2012). The posterior hypothalamus (PH) is heavily innervated by the IL, with a substantial proportion of the input targeting intrinsic GABAergic neurons (Myers et al., 2016). Chemical inhibition of the PH increases HPA axis stress responses, whereas excitation enhances HPA axis responses and induces PVN Fos activation. Importantly, local inactivation/activation studies indicate that the PH is also critical for driving behavioral stress responses (anxiety-like behavior) as well as autonomic responses to air puff (Lisa et al., 1989; Myers et al., 2016). Combined with evidence for recruitment of the PH by chronic stress, we hypothesized that the PH may comprise a proximal source of PVN drive linked to descending information on stressor chronicity, perhaps relayed by the IL (Fig. 19.2). The ILePH connection also appears to follow the general rule of stress information flow through interneurons. Tracing studies indicating strong input from the IL onto PH gamma aminobutyric acid (GABA) neurons suggest that the IL mediates stress response inhibition by activating intrinsic excitatory neurons in the PH, which are known to provide strong glutamatergic input to the PVN (and presumably other stress effectors systems) (Myers et al., 2016; Ulrich-Lai et al., 2011). The NTS is also known to receive descending input from the IL, as well as other limbic stress regulatory regions such as the CeA (Schwaber et al., 1982; van der Kooy et al., 1984). The NTS is known to provide rich noradrenergic as well as nonnoradrenergic (e.g., glucagon-like peptide-1 [GLP-1]) input to the PVN (Cunningham and Sawchenko, 1988; Tauchi et al., 2008b). PVN-projecting norepinephrine (NE) neurons appear to be responsible for acute responses to systemic stressors, but they do not appear to be important in mediating effects of chronic stress. For example, selective lesion of PVN-projecting NE neurons in the NTS blocks responses to metabolic stress (2-DG) but not swim stress (Ritter et al., 2003). Thus, ascending NTS NE inputs likely convey input from interoceptors on homeostatic challenge, but they do not appear to interface with cognitive processing of stress in the forebrain. Similarly, immunolesion of PVN-projecting NE neurons with dopamine beta-hydroxylase antibodyesaporin conjugates attenuates acute stress reactivity (interestingly, to restraint), but it does not inhibit somatic or HPA axis responses to stress in any simple way (Flak et al., 2014).

Toward a neurocircuitry of stress resilience

303

In contrast, pharmacological inhibition of PVN GLP-1 receptors blocks ACTH and corticosterone responses to both systemic (LICl) and psychogenic (novelty) stressors (Kinzig et al., 2003), indicative of a more general role for this cell population in HPA axis integration. This effect is recapitulated in mouse by genetic deletion of GLP-1r in PVN neurons (using Sim1 Cre recombinase driver lines crossed with Glp1r flox mice), where loss of GLP-1 signaling reduces HPA axis responses to both restraint and hypoxia (Ghosal et al., 2017). Importantly, neurons in the NTS are likely the sole source of GLP-1 projections to the PVN (Trapp and Richards, 2013). These data implicate NTS GLP-1 neurons in coordination with HPA axis responses across modalities and may be mediated by afferent input from descending corticolimbic neurons, perhaps from regions such as the IL (or CeA). Ascending GLP-1 neurons are also implicated in chronic stress sensitization of the HPA axis. Central (intracerebroventricular) injections of GLP-1 exagerate the effects of chronic variable stress on HPA axis sensitization, whereas prestressor treatment with antagonist (dHG-exendin) blocks chronic sensitization (Tauchi et al., 2008a). In mice, Sim1-directed deletion of PVN GLP1-R also inhibited corticosterone secretion following chronic variable stress (Ghosal et al., 2017). Notably, both intracerebroventricular GLP-1r antagonist and PVN GLP-1r knockout decrease chronic stresseinduced weight reduction, suggesting a link between the ascending GLP-1 system and metabolic consequences of stress (Ghosal et al., 2017; Tauchi et al., 2008a) (Fig. 19.2). In keeping with a recurring theme, glucocorticoid signaling is poised to play a role in stress regulation by the NTS. Stress regulatory GLP-1 neurons are targeted by glucocorticoids. Acute or chronic stress downregulates expression of preproglucagon, the GLP-1 precursor protein, in a glucocorticoid-dependent manner (Zhang et al., 2010; Zhang et al., 2009). Local implants of the GR antagonist mifepristone exaggerate somatic and HPA axis responses to chronic stress exposure, suggesting that glucocorticoids released during stress are required for optimal control of chronic stress adaptation at the NTS (Ghosal et al., 2014). Glucocorticoids may thus precipitate a loss of GLP-1 signaling capacity, which then reduces drive of the HPA axis consequent to acute and potentially chronic drive. It is notable that both the PH and NTS are implicated in chronic stress integration, and both regions connect with the IL (Myers et al., 2016; Vertes, 2004). Thus, these regions have the capacity to work together either in parallel or possibly in series. Additional work is required to delineate whether this circuitry plays a pivotal role in stress regulation.

Toward a neurocircuitry of stress resilience The literature summarized above casts light on some key features that regulate stress reactivity during chronic stress and likely drive “resilience” to the impact of chronic stress: 1. Stress regulatory circuits can differentially affect short-term versus long-term adaptation. From a circuit perspective, chronic stress is not acute stress, prolonged. It is clear that the structures that are critical for acute stress adaptation may not be involved in control of stress in the chronic time realm or indeed may play different roles in the response to prolonged drive. This appears to be the case for the hippocampus/SUBv, which inhibits HPA axis responses to acute psychogenic stress but does not affect measures of chronic

304

19. Corticolimbic stress regulatory circuits, hypothalamoepituitaryeadrenocortical adaptation, and resilience

drive; the MeA, which drives psychogenic responses but only affects weight change under chronic stress conditions; and the IL, which seems to limit HPA drive following both acute and chronic stress. Some regions may even play different roles in the context of acute versus chronic stress, e.g., lesions of the anterior BST reduce HPA axis responses to acute psychogenic stress but enhance responses following chronic stress exposure. 2. Corticolimbic projections converge on subcortical relays and regulate output in an additive (or perhaps also subtractive) fashion. There is evidence of convergence among corticolimbic stressemodulatory circuits onto subcortical targets. This offers a potential interface whereby information coming from diverse sites can be summated to optimize activity of descending relays (e.g., BST to PVN connections). Hence, there is evidence for cooperation among stress integrative circuits to drive adaptation. 3. Stressor chronicity is likely coordinated by corticolimbic interactions with the thalamus. The midline thalamus (PVT) appears to play an important role in reading and communicating stressor chronicity. Extensive bidirectional connectivity of this region with corticolimbic regulatory sites and downstream stress effectors may permit coordination of output with respect to prior experience. Given the substantial reciprocal arrangement of the PVT and all relevant corticolimbic sites noted above, it is difficult to say whether PVT coordinates the respective outputs of corticolimbic nodes or serves as a relay to downstream structures proximal to the PVN (or other stress effector pathways). 4. Glucocorticoid signaling plays a modulatory role in control of regions that feed into stress adaptation. Hence, the HPA axis itself provides a contextual signal that informs corticolimbic and subcortical integrators as to how the physiological reaction to stress is proceeding in the body. The adjustments the brain makes to these signals likely play a major role in conferring situationally matched physiological and likely psychological reactions and decisions. These features have implications for resilience. In aggregate, the literature indicates that numerous corticolimbic regions receive sensory, associational, and hormonal (e.g., glucocorticoid) inputs that are processed in parallel, on the basis of the specific functional domains of each region. All of the regions then have the opportunity to “have their say” with respect to outcome via processing at downstream nodes responsible for integrated responses. Net results with respect to stress outcomes (i.e., HPA axis responses) are gated by inhibitory circuits that are either reinforced or disinhibited by summated inputs from upstream corticolimbic neurons. Resilience is afforded by two mechanisms, likely at work in each region: an acute regulatory response promoting resilience to an immediate threat and chronic responses that control long-term resilience. Chronic stress responses may augment or oppose those engendered by the acute response, based on input from chronicity circuitry (e.g., the PVT). It should not be lost on the reader that I have not really provided a unified theory of adaptation and resilience, but rather pointed out how understanding of stress circuitry has become more complex with consideration of chronicity. Indeed, understanding of circuit regulation of the HPA axis was much simpler when we monitored only the acute stress response. Moreover, the nature of the data from the single reporter of a common final pathway response (HPA axis drive) made us far too comfortable with the erroneous assumption that similar circuitry should subserve acute and chronic stress, just along a different time line.

References

305

As chronicity likely drives pathology, understanding how stress integration and perception advances from “acute” adaptive responses to “chronic” changes and potential pathology will likely be a topic of intense scrutiny in the field. A key point of resilience is the ability to resist this transition to stress maladaptation, and thus identifying the process at the core of this transition will be pivotal to developing approaches to confer resilience in those who have lost it or indeed never possessed it.

Acknowledgments I would like to thank past and present Herman lab members for their invaluable contributions to this work and our current and past funding from the NIMH (MH049698, MH101729, MH069860).

References Bhatnagar, S., Dallman, M., 1998. Neuroanatomical basis for facilitation of hypothalamic-pituitary-adrenal responses to a novel stressor after chronic stress. Neuroscience 84 (4), 1025e1039. Bhatnagar, S., Huber, R., Nowak, N., Trotter, P., 2002. Lesions of the posterior paraventricular thalamus block habituation of hypothalamic-pituitary-adrenal responses to repeated restraint. Journal of Neuroendocrinology 14 (5), 403e410. Bhatnagar, S., Vining, C., Denski, K., 2004. Regulation of chronic stress-induced changes in hypothalamic-pituitaryadrenal activity by the basolateral amygdala. Annals of the New York Academy of Sciences 1032, 315e319. https://doi.org/10.1196/annals.1314.050. Boyle, M.P., Brewer, J.A., Funatsu, M., Wozniak, D.F., Tsien, J.Z., Izumi, Y., Muglia, L.J., 2005. Acquired deficit of forebrain glucocorticoid receptor produces depression-like changes in adrenal axis regulation and behavior. Proceedings of the National Academy of Sciences of the United States of America 102 (2), 473e478. Canteras, N.S., Simerly, R.B., Swanson, L.W., 1995. Organization of projections from the medial nucleus of the amygdala: a PHAL study in the rat. The Journal of Comparative Neurology 360 (2), 213e245. Chiba, S., Numakawa, T., Ninomiya, M., Richards, M.C., Wakabayashi, C., Kunugi, H., 2012. Chronic restraint stress causes anxiety- and depression-like behaviors, downregulates glucocorticoid receptor expression, and attenuates glutamate release induced by brain-derived neurotrophic factor in the prefrontal cortex. Progress In Neuro-Psychopharmacology and Biological Psychiatry 39 (1), 112e119. Choi, D.C., Evanson, N.K., Furay, A.R., Ulrich-Lai, Y.M., Ostrander, M.M., Herman, J.P., 2008a. The anteroventral bed nucleus of the stria terminalis differentially regulates hypothalamic-pituitary-adrenocortical axis responses to acute and chronic stress. Endocrinology 149 (2), 818e826. Choi, D.C., Furay, A.R., Evanson, N.K., Ostrander, M.M., Ulrich-Lai, Y.M., Herman, J.P., 2007. Bed nucleus of the stria terminalis subregions differentially regulate hypothalamic-pituitary-adrenal axis activity: implications for the integration of limbic inputs. Journal of Neuroscience 27 (8), 2025e2034. Choi, D.C., Furay, A.R., Evanson, N.K., Ulrich-Lai, Y.M., Nguyen, M.M., Ostrander, M.M., Herman, J.P., 2008b. The role of the posterior medial bed nucleus of the stria terminalis in modulating hypothalamic-pituitary-adrenocortical axis responsiveness to acute and chronic stress. Psychoneuroendocrinology 33 (5), 659e669. Cook, C.J., 2002. Glucocorticoid feedback increases the sensitivity of the limbic system to stress. Physiology and Behavior 75 (4), 455e464. Cullinan, W.E., Helmreich, D.L., Watson, S.J., 1996. Fos expression in forebrain afferents to the hypothalamic paraventricular nucleus following swim stress. The Journal of Comparative Neurology 368 (1), 88e99. Cullinan, W.E., Herman, J.P., Battaglia, D.F., Akil, H., Watson, S.J., 1995. Pattern and time course of immediate early gene expression in rat brain following acute stress. Neuroscience 64, 477e505. Cullinan, W.E., Herman, J.P., Watson, S.J., 1993. Ventral subicular interaction with the hypothalamic paraventricular nucleus: evidence for a relay in the bed nucleus of the stria terminalis. The Journal of Comparative Neurology 332, 1e20. Cunningham Jr., E.T., Sawchenko, P.E., 1988. Anatomical specificity of noradrenergic inputs to the paraventricular and supraoptic nuclei of the rat hypothalamus. The Journal of Comparative Neurology 274 (1), 60e76. https://doi.org/10.1002/cne.902740107.

306

19. Corticolimbic stress regulatory circuits, hypothalamoepituitaryeadrenocortical adaptation, and resilience

Day, H.E., Masini, C.V., Campeau, S., 2004. The pattern of brain c-fos mRNA induced by a component of fox odor, 2,5-dihydro-2,4,5-trimethylthiazoline (TMT), in rats, suggests both systemic and processive stress characteristics. Brain Research 1025 (1e2), 139e151. https://doi.org/10.1016/j.brainres.2004.07.079. Dayas, C.V., Buller, K.M., Crane, J.W., Xu, Y., Day, T.A., 2001. Stressor categorization: acute physical and psychological stressors elicit distinctive recruitment patterns in the amygdala and in medullary noradrenergic cell groups. European Journal of Neuroscience 14 (7), 1143e1152. Dayas, C.V., Buller, K.M., Day, T.A., 1999. Neuroendocrine responses to an emotional stressor: evidence for involvement of the medial but not the central amygdala. European Journal of Neuroscience 11 (7), 2312e2322. de Kloet, A.D., Herman, J.P., 2018. Fat-brain connections: adipocyte glucocorticoid control of stress and metabolism. Frontiers in Neuroendocrinology 48, 50e57. https://doi.org/10.1016/j.yfrne.2017.10.005. de Kloet, E.R., Joels, M., Holsboer, F., 2005. Stress and the brain: from adaptation to disease. Nature Reviews Neuroscience 6 (6), 463e475. https://doi.org/10.1038/nrn1683. De Kloet, E.R., Vreugdenhil, E., Oitzl, M.S., Joels, M., 1998. Brain corticosteroid receptor balance in health and disease. Endocrine Reviews 19 (3), 269e301. https://doi.org/10.1210/edrv.19.3.0331. Diorio, D., Viau, V., Meaney, M.J., 1993. The role of the medial prefrontal cortex (cingulate gyrus) in the regulation of hypothalamo-pituitary-adrenal responses to stress. Journal of Neuroscience 13, 3839e3847. Dong, H.W., Swanson, L.W., 2004. Organization of axonal projections from the anterolateral area of the bed nuclei of the stria terminalis. The Journal of Comparative Neurology 468 (2), 277e298. https://doi.org/10.1002/cne.10949. Dong, H.W., Swanson, L.W., 2006a. Projections from bed nuclei of the stria terminalis, anteromedial area: cerebral hemisphere integration of neuroendocrine, autonomic, and behavioral aspects of energy balance. The Journal of Comparative Neurology 494 (1), 142e178. https://doi.org/10.1002/cne.20788. Dong, H.W., Swanson, L.W., 2006c. Projections from bed nuclei of the stria terminalis, dorsomedial nucleus: implications for cerebral hemisphere integration of neuroendocrine, autonomic, and drinking responses. The Journal of Comparative Neurology 494 (1), 75e107. https://doi.org/10.1002/cne.20790. Emmert, M.H., Herman, J.P., 1999. Differential forebrain c-fos mRNA induction by ether inhalation and novelty: evidence for distinctive stress pathways. Brain Research 845 (1), 60e67. Feldman, S., Conforti, N., Itzik, A., Weidenfeld, J., 1994. Differential effect of amygdaloid lesions of CRF-41, ACTH and corticosterone responses following neural stimuli. Brain Research 658, 21e26. Feldman, S., Conforti, N., Saphier, D., 1990. The preoptic area and bed nucleus of the stria terminalis are involved in the effects of the amygdala on adrenocortical secretion. Neuroscience 37 (3), 775e779. Figueiredo, H.F., Bodie, B.L., Tauchi, M., Dolgas, C.M., Herman, J.P., 2003. Stress integration after acute and chronic predator stress: differential activation of central stress circuitry and sensitization of the hypothalamo-pituitaryadrenocortical axis. Endocrinology 144 (12), 5249e5258. https://doi.org/10.1210/en.2003-0713. Flak, J.N., Myers, B., Solomon, M.B., McKlveen, J.M., Krause, E.G., Herman, J.P., 2014. Role of paraventricular nucleus-projecting norepinephrine/epinephrine neurons in acute and chronic stress. European Journal of Neuroscience 39 (11), 1903e1911. https://doi.org/10.1111/ejn.12587. Flak, J.N., Solomon, M.B., Jankord, R., Krause, E.G., Herman, J.P., 2012. Identification of chronic stress-activated regions reveals a potential recruited circuit in rat brain. European Journal of Neuroscience 36 (4), 2547e2555. https://doi.org/10.1111/j.1460-9568.2012.08161.x. Furay, A.R., Bruestle, A.E., Herman, J.P., 2008. The role of the forebrain glucocorticoid receptor in acute and chronic stress. Endocrinology 149, 5482e5490. https://doi.org/10.1210/en.2008-0642. Ghosal, S., Bundzikova-Osacka, J., Dolgas, C.M., Myers, B., Herman, J.P., 2014. Glucocorticoid receptors in the nucleus of the solitary tract (NTS) decrease endocrine and behavioral stress responses. Psychoneuroendocrinology 45, 142e153. https://doi.org/10.1016/j.psyneuen.2014.03.018. Ghosal, S., Packard, A.E.B., Mahbod, P., McKlveen, J.M., Seeley, R.J., Myers, B., et al., 2017. Disruption of glucagonlike peptide 1 signaling in Sim1 neurons reduces physiological and behavioral reactivity to acute and chronic stress. Journal of Neuroscience 37 (1), 184e193. https://doi.org/10.1523/JNEUROSCI.1104-16.2016. Gold, P.W., Chrousos, G.P., 1999. The endocrinology of melancholic and atypical depression: relation to neurocircuitry and somatic consequences. Proceedings of the Association of American Physicians 111 (1), 22e34. Grissom, N.M., Bhatnagar, S., 2011. The basolateral amygdala regulates adaptation to stress via beta-adrenergic receptor-mediated reductions in phosphorylated extracellular signal-regulated kinase. Neuroscience 178, 108e122. https://doi.org/10.1016/j.neuroscience.2010.12.049.

References

307

Herman, J.P., Adams, D., Prewitt, C., 1995. Regulatory changes in neuroendocrine stress-integrative circuitry produced by a variable stress paradigm. Neuroendocrinology 61 (2), 180e190. Herman, J.P., Cullinan, W.E., 1997. Neurocircuitry of stress: central control of the hypothalamo-pituitary-adrenocortical axis. Trends in Neurosciences 20 (2), 78e84. Herman, J.P., Dolgas, C.M., Carlson, S.L., 1998. Ventral subiculum regulates hypothalamo-pituitary-adrenocortical and behavioural responses to cognitive stressors. Neuroscience 86 (2), 449e459. Herman, J.P., Figueiredo, H., Mueller, N.K., Ulrich-Lai, Y., Ostrander, M.M., Choi, D.C., Cullinan, W.E., 2003. Central mechanisms of stress integration: hierarchical circuitry controlling hypothalamo-pituitary-adrenocortical responsiveness. Frontiers in Neuroendocrinology 24 (3), 151e180. Herman, J.P., McKlveen, J.M., Ghosal, S., Kopp, B., Wulsin, A., Makinson, R., et al., 2016. Regulation of the hypothalamic-pituitary-adrenocortical stress response. Comparative Physiology 6 (2), 603e621. https:// doi.org/10.1002/cphy.c150015. Herman, J.P., Mueller, N.K., 2006. Role of the ventral subiculum in stress integration. Behavioural Brain Research 174 (2), 215e224. https://doi.org/10.1016/j.bbr.2006.05.035. Hill, M.N., McLaughlin, R.J., Pan, B., Fitzgerald, M.L., Roberts, C.J., Lee, T.T., et al., 2011. Recruitment of prefrontal cortical endocannabinoid signaling by glucocorticoids contributes to termination of the stress response. Journal of Neuroscience 31 (29), 10506e10515. https://doi.org/10.1523/JNEUROSCI.0496-11.2011. Jacobson, L., Sapolsky, R., 1991. The role of the hippocampus in feedback regulation of the hypothalamic-pituitaryadrenocortical axis. Endocrine Reviews 12 (2), 118e134. https://doi.org/10.1210/edrv-12-2-118. Jaferi, A., Nowak, N., Bhatnagar, S., 2003. Negative feedback functions in chronically stressed rats: role of the posterior paraventricular thalamus. Physiology and Behavior 78 (3), 365e373. John, C.D., Christian, H.C., Morris, J.F., Flower, R.J., Solito, E., Buckingham, J.C., 2004. Annexin 1 and the regulation of endocrine function. Trends in Endocrinology and Metabolism 15 (3), 103e109. https://doi.org/10.1016/ j.tem.2004.02.001. Jones, K.R., Myers, B., Herman, J.P., 2011. Stimulation of the prelimbic cortex differentially modulates neuroendocrine responses to psychogenic and systemic stressors. Physiology and Behavior 104 (2), 266e271. Keller-Wood, M., Dallman, M.F., 1984. Corticosteroid inhibition of ACTH secretion. Endocrine Reviews 5 (1), 1e24. Kim, S.H., Han, J.E., Hwang, S., Oh, D.H., 2010. The expression of corticotropin-releasing factor in the central nucleus of the amygdala, induced by colorectal distension, is attenuated by general anesthesia. Journal of Korean Medical Science 25 (11), 1646e1651. https://doi.org/10.3346/jkms.2010.25.11.1646. Kinzig, K.P., D’Alessio, D.A., Herman, J.P., Sakai, R.R., Vahl, T.P., Figueiredo, H.F., et al., 2003. CNS glucagon-like peptide-1 receptors mediate endocrine and anxiety responses to interoceptive and psychogenic stressors. Journal of Neuroscience 23 (15), 6163e6170. Kollack-Walker, S., Newman, S.W., 1995. Mating and agonistic behavior produce different patterns of Fos immunolabeling in the male Syrian hamster brain. Neuroscience 66 (3), 721e736. Langgartner, D., Fuchsl, A.M., Uschold-Schmidt, N., Slattery, D.A., Reber, S.O., 2015. Chronic subordinate colony housing paradigm: a mouse model to characterize the consequences of insufficient glucocorticoid signaling. Frontiers in Psychiatry 6, 18. https://doi.org/10.3389/fpsyt.2015.00018. Langgartner, D., Peterlik, D., Foertsch, S., Fuchsl, A.M., Brokmann, P., Flor, P.J., et al., 2017. Individual differences in stress vulnerability: the role of gut pathobionts in stress-induced colitis. Brain, Behavior, and Immunity 64, 23e32. https://doi.org/10.1016/j.bbi.2016.12.019. Li, S., Kirouac, G.J., 2012. Sources of inputs to the anterior and posterior aspects of the paraventricular nucleus of the thalamus. Brain Structure and Function 217 (2), 257e273. https://doi.org/10.1007/s00429-011-0360-7. Lisa, M., Marmo, E., Wible Jr., J.H., DiMicco, J.A., 1989. Injection of muscimol into posterior hypothalamus blocks stress-induced tachycardia. American Journal of Physiology 257 (1 Pt 2), R246eR251. https://doi.org/10.1152/ ajpregu.1989.257.1.R246. Liu, J., Garza, J.C., Li, W., Lu, X.Y., 2013. Melanocortin-4 receptor in the medial amygdala regulates emotional stressinduced anxiety-like behaviour, anorexia and corticosterone secretion. The International Journal of Neuropsychopharmacology 16 (1), 105e120. https://doi.org/10.1017/S146114571100174X. Makino, S., Gold, P.W., Schulkin, J., 1994. Corticosterone effects on corticotropin-releasing hormone mRNA in the central nucleus of the amygdala and the parvocellular region of the paraventricular nucleus of the hypothalamus. Brain Research 640 (1e2), 105e112.

308

19. Corticolimbic stress regulatory circuits, hypothalamoepituitaryeadrenocortical adaptation, and resilience

Makino, S., Shibasaki, T., Yamauchi, N., Nishioka, T., Mimoto, T., Wakabayashi, I., et al., 1999. Psychological stress increased corticotropin-releasing hormone mRNA and content in the central nucleus of the amygdala but not in the hypothalamic paraventricular nucleus in the rat. Brain Research 850 (1e2), 136e143. McKlveen, J.M., Morano, R.L., Fitzgerald, M., Zoubovsky, S., Cassella, S.N., Scheimann, J.R., et al., 2016. Chronic stress increases prefrontal inhibition: a mechanism for stress-induced prefrontal dysfunction. Biological Psychiatry 80 (10), 754e764. https://doi.org/10.1016/j.biopsych.2016.03.2101. McKlveen, J.M., Myers, B., Flak, J.N., Bundzikova, J., Solomon, M.B., Seroogy, K.B., Herman, J.P., 2013. Role of prefrontal cortex glucocorticoid receptors in stress and emotion. Biological Psychiatry 74 (9), 672e679. https:// doi.org/10.1016/j.biopsych.2013.03.024. Munck, A., Guyre, P.M., Holbrook, N.J., 1984. Physiological functions of glucocorticoids in stress and their relation to pharmacological actions. Endocrine Reviews 5 (1), 25e44. https://doi.org/10.1210/edrv-5-1-25. Myers, B., Carvalho-Netto, E., Wick-Carlson, D., Wu, C., Naser, S., Solomon, M.B., et al., 2016. GABAergic signaling within a limbic-hypothalamic circuit integrates social and anxiety-like behavior with stress reactivity. Neuropsychopharmacology 41 (6), 1530e1539. https://doi.org/10.1038/npp.2015.311. Myers, B., McKlveen, J.M., Herman, J.P., 2012. Neural regulation of the stress response: the many faces of feedback. Cellular and Molecular Neurobiology. https://doi.org/10.1007/s10571-012-9801-y. Myers, B., McKlveen, J.M., Morano, R., Ulrich-Lai, Y.M., Solomon, M.B., Wilson, S.P., Herman, J.P., 2017. Vesicular glutamate transporter 1 knockdown in infralimbic prefrontal cortex augments neuroendocrine responses to chronic stress in male rats. Endocrinology 158 (10), 3579e3591. https://doi.org/10.1210/en.2017-00426. Ongur, D., Price, J.L., 2000. The organization of networks within the orbital and medial prefrontal cortex of rats, monkeys and humans. Cerebral Cortex 10 (3), 206e219. Ostrander, M.M., Richtand, N.M., Herman, J.P., 2003. Stress and amphetamine induce Fos expression in medial prefrontal cortex neurons containing glucocorticoid receptors. Brain Research 990 (1e2), 209e214. Prewitt, C.M., Herman, J.P., 1997. Hypothalamo-pituitary-adrenocortical regulation following lesions of the central nucleus of the amygdala. Stress: The International Journal on the Biology of Stress 1 (4), 263e280. Prewitt, C.M., Herman, J.P., 1998. Anatomical interactions between the central amygdaloid nucleus and the hypothalamic paraventricular nucleus of the rat: a dual tract-tracing analysis. Journal of Chemical Neuroanatomy 15 (3), 173e185. Radley, J.J., Arias, C.M., Sawchenko, P.E., 2006. Regional differentiation of the medial prefrontal cortex in regulating adaptive responses to acute emotional stress. Journal of Neuroscience 26 (50), 12967e12976. Radley, J.J., Sawchenko, P.E., 2011. A common substrate for prefrontal and hippocampal inhibition of the neuroendocrine stress response. Journal of Neuroscience 31 (26), 9683e9695. Radley, J.J., Sawchenko, P.E., 2015. Evidence for involvement of a limbic paraventricular hypothalamic inhibitory network in hypothalamic-pituitary-adrenal axis adaptations to repeated stress. The Journal of Comparative Neurology 523 (18), 2769e2787. https://doi.org/10.1002/cne.23815. Ritter, S., Watts, A.G., Dinh, T.T., Sanchez-Watts, G., Pedrow, C., 2003. Immunotoxin lesion of hypothalamically projecting norepinephrine and epinephrine neurons differentially affects circadian and stressor-stimulated corticosterone secretion. Endocrinology 144 (4), 1357e1367. https://doi.org/10.1210/en.2002-221076. Roozendaal, B., Koolhaas, J.M., Bohus, B., 1990. Differential effect of lesioning of the central amygdala on the bradycardiac and behavioral response of the rat in relation to conditioned social and solitary stress. Behavioural Brain Research 41 (1), 39e48. Schwaber, J.S., Kapp, B.S., Higgins, G.A., P.R, R., 1982. Amygdala and basal forebrain direct connections with the nucleus of the solitary tract and the dorsal motor nucleus. Journal of Neuroscience 2, 1424e1438. Solomon, M.B., Jones, K., Packard, B.A., Herman, J.P., 2010. The medial amygdala modulates body weight but not neuroendocrine responses to chronic stress. Journal of Neuroendocrinology 22 (1), 13e23. https://doi.org/ 10.1111/j.1365-2826.2009.01933.x. Sullivan, R.M., Gratton, A., 1999. Lateralized effects of medial prefrontal cortex lesions on neuroendocrine and autonomic stress responses in rats. Journal of Neuroscience 19 (7), 2834e2840. Tasker, J.G., Herman, J.P., 2011. Mechanisms of rapid glucocorticoid feedback inhibition of the hypothalamicpituitary-adrenal axis. Stress: The International Journal on the Biology of Stress 14 (4), 398e406. https:// doi.org/10.3109/10253890.2011.586446.

References

309

Tauchi, M., Zhang, R., D’Alessio, D.A., Seeley, R.J., Herman, J.P., 2008a. Role of central glucagon-like peptide-1 in hypothalamo-pituitary-adrenocortical facilitation following chronic stress. Experimental Neurology 210 (2), 458e466. https://doi.org/10.1016/j.expneurol.2007.11.016. Tauchi, M., Zhang, R., D’Alessio, D.A., Stern, J.E., Herman, J.P., 2008b. Distribution of glucagon-like peptide-1 immunoreactivity in the hypothalamic paraventricular and supraoptic nuclei. Journal of Chemical Neuroanatomy 36 (3e4), 144e149. https://doi.org/10.1016/j.jchemneu.2008.07.009. Thrivikraman, K.V., Su, Y., Plotsky, P.M., 1997. Patterns of fos-immunoreactivity in the CNS induced by repeated hemorrhage in conscious rats: correlations with pituitary-adrenal axis activity. Stress: The International Journal on the Biology of Stress 2 (2), 145e158. Trapp, S., Richards, J.E., 2013. The gut hormone glucagon-like peptide-1 produced in brain: is this physiologically relevant? Current Opinion in Pharmacology 13 (6), 964e969. https://doi.org/10.1016/j.coph.2013.09.006. Traub, R.J., Silva, E., Gebhart, G.F., Solodkin, A., 1996. Noxious colorectal distention induced-c-Fos protein in limbic brain structures in the rat. Neuroscience Letters 215 (3), 165e168. Ulrich-Lai, Y.M., Christiansen, A.M., Ostrander, M.M., Jones, A.A., Jones, K.R., Choi, D.C., et al., 2010. Pleasurable behaviors reduce stress via brain reward pathways. Proceedings of the National Academy of Sciences of the United States of America 107 (47), 20529e20534. https://doi.org/10.1073/pnas.1007740107. Ulrich-Lai, Y.M., Herman, J.P., 2009. Neural regulation of endocrine and autonomic stress responses. Nature Reviews Neuroscience 10 (6), 397e409. Ulrich-Lai, Y.M., Jones, K.R., Ziegler, D.R., Cullinan, W.E., Herman, J.P., 2011. Forebrain origins of glutamatergic innervation to the rat paraventricular nucleus of the hypothalamus: differential inputs to the anterior versus posterior subregions. The Journal of Comparative Neurology 519 (7), 1301e1319. https://doi.org/10.1002/cne.22571. van der Kooy, D., Koda, L.Y., McGinty, J.F., Gerfen, C.R., Bloom, F.E., 1984. The organization of projections from the cortex, amygdala, and hypothalamus to the nucleus of the solitary tract in rat. The Journal of Comparative Neurology 224 (1), 1e24. Vertes, R.P., 2004. Differential projections of the infralimbic and prelimbic cortex in the rat. Synapse 51 (1), 32e58. https://doi.org/10.1002/syn.10279. Vertes, R.P., Hoover, W.B., 2008. Projections of the paraventricular and paratenial nuclei of the dorsal midline thalamus in the rat. The Journal of Comparative Neurology 508 (2), 212e237. https://doi.org/10.1002/cne.21679. Vidal-Gonzalez, I., Vidal-Gonzalez, B., Rauch, S.L., Quirk, G.J., 2006. Microstimulation reveals opposing influences of prelimbic and infralimbic cortex on the expression of conditioned fear. Learning and Memory 13 (6), 728e733. https://doi.org/10.1101/lm.306106. Willner, P., 2017. The chronic mild stress (CMS) model of depression: history, evaluation and usage. Neurobiol Stress 6, 78e93. https://doi.org/10.1016/j.ynstr.2016.08.002. Yehuda, R., 2002. Post-traumatic stress disorder. New England Journal of Medicine 346 (2), 108e114. https:// doi.org/10.1056/NEJMra012941. Zhang, R., Jankord, R., Flak, J.N., Solomon, M.B., D’Alessio, D.A., Herman, J.P., 2010. Role of glucocorticoids in tuning hindbrain stress integration. Journal of Neuroscience 30 (44), 14907e14914. https://doi.org/10.1523/JNEUROSCI.0522-10.2010. Zhang, R., Packard, B.A., Tauchi, M., D’Alessio, D.A., Herman, J.P., 2009. Glucocorticoid regulation of preproglucagon transcription and RNA stability during stress. Proceedings of the National Academy of Sciences of the United States of America 106 (14), 5913e5918. https://doi.org/10.1073/pnas.0808716106.

C H A P T E R

20

Biomarkers of resilience and susceptibility in rodent models of stress

1

Ricardo Magalhães1, 2, Edward Ganz1, 2, Mariana Rodrigues1, 2, 3, David André Barrière4, 5, Sébastien Mériaux6, Thérèse M. Jay4, 5, Nuno Sousa1, 2 Life and Health Sciences Research Institute (ICVS), School of Medicine, University of Minho, Braga, Portugal; 2ICVS/3B’s, PT Government Associate Laboratory, Braga/Guimarães, Portugal; 3Algoritmi Centre, University of Minho, Braga, Portugal; 4Physiopathologie des Maladies Psychiatriques, UMR_S 894 Inserm, Centre de Psychiatrie et Neurosciences, Paris, France; 5Faculté de Médecine Paris Descartes, Service Hospitalo-Universitaire, Centre Hospitalier Sainte-Anne, Paris, France; 6Neurospin, JOLIOT, CEA, Gif-sur-Yvette, France

Introduction In today’s society, exposure to stressful situations is inescapable and is a situation that we all must face. Although frequently spoken of as a modern phenomenon, stress has always played a key role as a pressure agent for natural selection, species adaptation, and evolution (Bijlsma and Loeschcke, 2005; Singh et al., 2008). Although such exposure to stressful stimuli and environment has been a constant in human evolution, the nature of the stressors has evolved with society (Jackson, 2014), and the ability to adapt to these new pressures plays a critical role in determining the fitness of each individual and species (Horne et al., 2014; Parsons, 1993). An overburden of, as well as the inability to cope with, stress has been associated with the development of a range of behavioral and medical disorders (de Kloet et al., 2005; McEwen, 2003). Some examples are depression (Hammen, 2005; Melchior et al., 2007; Southwick and Charney, 2012; Welberg, 2014; Kendler et al., 1999), panic, and anxiety disorders (Pego et al., 2008; McEwen et al., 2012), in which stress can be considered a primary cause, and

Stress Resilience https://doi.org/10.1016/B978-0-12-813983-7.00020-3

311

Copyright © 2020 Elsevier Inc. All rights reserved.

312

20. Biomarkers of resilience and susceptibility in rodent models of stress

others, such as schizophrenia (Walker et al., 2008; Aiello et al., 2012), obsessive-compulsive disorder (Morgado et al., 2013; Toro et al., 1992; Findley et al., 2003), and Alzheimer’s disease (Donaldson et al., 1998; Sotiropoulos et al., 2008, 2011), where there is highly suggestive evidence that stress plays a key trigger role. Stress has also been implicated in disorders that are not primarily associated with the central nervous system (CNS) but rather manifest in the cardiac, immune, and digestive systems (O’Mahony et al., 2009; Jones et al., 2006). In addition to understanding the global impact of stress on the CNS, it is important to consider both the spectrum of individual responses to the same stressful stimulus or environment, as well as to determine how a response develops as a function of time (Sousa, 2016). A specific response may be characterized as either susceptible or resilient (Southwick and Charney, 2012) (alternatively, vulnerable or resistant). A resilient/resistant response is defined as that which enables an individual to arrive at some form of positive outcome (such as the maintenance of the homeostasis) in the face of stress (McEwen et al., 2015; Wu et al., 2013). In contradistinction, the response of a susceptible/vulnerable individual is negative or maladaptive. It is hoped that by developing methods able to identify subjects susceptible to overall stress, or to specific stressors, it will be possible to identify those at risk of developing the associated disorders (Russo et al., 2012). A cornerstone of understanding the individual responses to stress is the ability to identify appropriate biomarkers. The term biomarker may be considered in several contexts (Strimbu and Tavel, 2010) but broadly can be defined as an objectively measurable variable that reflects a biological, pathogenic, or pharmacological process (Biomarkers Definitions Working, 2001). Some examples of currently used biomarkers are measurable behavior, assayable serum levels of a specific metabolite or transmitter, and parameters of physiological activity. The choice of appropriate biomarkers is complex and is governed by how they are to be used. When considering potential clinical applicability, the sensitivity and specificity of the marker are of primary concern. With massive growth in the field of noninvasive imaging, particularly with continuing advances in all aspects of magnetic resonance imaging (MRI), there is great interest in identifying biomarkers, which can be measured with imaging techniques. It is to be hoped that the development and refinement of such capabilities will allow faster and more accurate diagnosis, as well as enable earlier identification of patients susceptible to diseases in which stress plays a role. Herein, we briefly review and discuss a number of approaches to the use of biomarkers that have been described in the literature and which offer the promise of being able to delineate the intrinsic differences between groups of responders to stress as a function of the markers chosen.

Experimental strategies One way to analyze the literature of stress resilience in animals is to characterize the experimental strategies that have been used. Herein, we classified the studies as either prospective or retrospective. In the former, the investigator identifies a specific variable or variables (e.g., genetic strain of the animal or rearing conditions) expected to mediate different responses to stress and then evaluates the ability of measurable markers, such as behavior or molecular

Experimental strategies

313

species to validate the observed differences, as well as to determine the underlying mechanisms. In the latter, only outcomes are considered, and solely analyzing the characteristics of the output function leads to the categorization of experimental subjects. Since the choice of experimental approach will lead to very different kinds of results, it is important that the most appropriate paradigm for the question posed be selected. Prospective strategies will be more valuable in addressing the role of specific stressors, stimuli, or treatments, while those of retrospective design are better suited for investigating broader, less specific, and potentially unanticipated responses and associations.

Prospective strategies A prospective study is designed to evaluate differing responses to stress as a function of a predetermined hypothesis. Such a strategy has three main advantages: (i) it allows for the spontaneous emergence of distinct responses within the study, thus allowing a clearer separation of phenotypes rather than generating a spectrum of responses; (ii) it enables the testing of specific hypotheses and mechanisms; and possibly most significant, (iii) its design is consistent with the testing of possible biomarker candidates potentially capable of identifying susceptible subjects before any exposure to stress. Such an approach still is dependent, however, on the use of appropriate biomarkers to retrospectively validate the initial grouping of the subjects. In prospective experimental designs, the outcome of susceptibility versus resilience is measured by the variable defined as the response and the experimental groups are defined a priori. For example, early life environment (such as prenatal stress and rearing conditions) has been hypothesized to play a critical role in the subsequent development, during adulthood, of either susceptible or resilient responses to stress. Various studies have used a number of different strategies to assess the role of early life conditions in the development of differing phenotypes. Variables reported include fostering conditions (Ladd et al., 2005), variation with genetic strain (Uchida et al., 2010), prenatal and early life stress, and adversity (Santarelli et al., 2017; Montes et al., 2016; Faraji et al., 2017), and even genetic variations in animals of the same strain from different vendors (Theilmann et al., 2016). A different sort of prospective approach was taken by Xu et al. (2016), who varied light exposure cycles to determine how photoperiod affects the development of the responses. Another group (Hodes et al., 2015) studied the susceptibility differences between male and female rats. It is a common strategy to compare rodent strains that are known to have different responses to the same stressor (Uchida et al., 2008, 2010; Magalhaes et al., 2017; Bourgin et al., 2015). Such an approach is useful to accentuate the differences in the responders, but it is then incumbent on the investigators to demonstrate clearly that the emergent differences actually result from the interaction between the animal and the stressor and not simply from the fact that the strains of the animals differ. A closely related issue is whether mechanisms delineated using such an approach are reproducible in within-individual strains. An alternative approach often used to group subjects is to assess behavior before the stress exposure. Clinton et al. (2014) used multiple lines of SpragueeDawley rats, known to have differing responses to novelty, to study stress-induced defecation. They found that low novelty-seeking animals, when compared with those that were high novelty

314

20. Biomarkers of resilience and susceptibility in rodent models of stress

seeking, demonstrated increased stress-induced defecation, while no differences were found in anxious-like behavior. On the other hand, Castro et al. (2012) showed that a combination of anxious-like and exploratory traits, as assessed before stress exposure, could largely predict and explain the resilient/susceptible stratification of these subjects. Another behavioral marker that has been used to identify susceptible animals before stress exposure is the existence of previous social isolation behaviors (Faraji et al., 2017). Finally, the prospective approach also has been a useful method in the study of whether gene expression or knockout (Varney et al., 2015; Nasca et al., 2015; Issler et al., 2014), or specific treatments, such as supplementing drinking water with corticosteroids (Macri et al., 2009), leads to differing responses.

Retrospective strategies Retrospective studies do not impose a specific model or grouping on a homogeneous population before the exposure to stress but rather seek to characterize the consequent response to that stress. The strength of such approaches is that they allow an unbiased search for any significant correlations in the global output of the system. They do not, however, allow for the testing of underlying mechanisms and are thus unable to establish causal relationships. In the retrospective study of susceptibility and resilience, the fundamental challenge is to determine which subjects are more susceptible and which are less so. The biomarkers most commonly used for this determination are behavioral. The behavioral phenomena that are analyzed are often observable analogues of human symptoms and include depressive-like behavior as analyzed through the forced swim test (Yang et al., 2015) to test the learned helplessness, anedhonia determined with the sucrose preference and consumption tests (Bergstrom et al., 2007; Christensen et al., 2011; Febbraro et al., 2017; Henningsen et al., 2012; Shao et al., 2013), behavioral consequences of social defeat and interactions (Anacker et al., 2016; Chen et al., 2015; Goto et al., 2016; Hodes et al., 2014; Isingrini et al., 2016; Jochems et al., 2015; Krishnan et al., 2007; Kumar et al., 2014; Pearson-Leary et al., 2017; Riga et al., 2017; Wood et al., 2010, 2015; Elliott et al., 2010), anxious-like behavior measured through the elevated plus maze (EPM) (Cohen et al., 2014), acoustic startle response (ASR) (Cohen et al., 2014), light-dark box (Nasca et al., 2015) and open field tests (Langgartner et al., 2017; Matrov et al., 2016), fear extinction in a long-term extinction recall (Reznikov et al., 2015), escape attempts from a electrical shock escape test (Benatti et al., 2012; Yang et al., 2016), and a swim escape test (Levay et al., 2006; Stiller et al., 2011). A significant number of the studies described in the literature focused on social behavior and interactions and reflected the particular nature of the stressors used in those experiments, many of which were paradigms of social defeat. Furthermore, most studies focus on only a single measure of behavior. As a result, it is reasonable to argue that the protocols of such experiments are capable of revealing only one component of a more global stress response. For this reason, the studies as a group are vulnerable to challenges on the basis of the generalizability of the findings, as well as the potential contribution of other confounds that may also affect the results. In all the referenced studies, criteria have been defined to stratify the population of stressed subjects into two or three subgroups. However, with the exception of those tests that happen to yield clearly bimodal distributions of

Potential additional biomarkers

315

responses (Benatti et al., 2012), the application of criteria that are subjective and a function of the investigator's choice, to partition what is actually a continuum of responses, would be expected to significantly affect the results and, thereby, the conclusions. In contrast to the above methods, some studies have explored the use of a combinatory approach and have used more than one marker in conjunction with grouping algorithms of varying complexity. Two studies (Cohen et al., 2014; Lebow et al., 2012), for example, have combined several behavior tests (EPM and ASR in Cohen et al., 2014 and a combination of five tests in Lebow et al., 2012) with a scoring system that attributes points according to each test response. The scores are then used to subdivide the subjects. Riga et al. (2017) combined a social approacheavoidance test with an object-place recognition paradigm using a two-step clustering analysis. Application of this technique made it possible for the investigators to identify depression-vulnerable and resilient subjects and subsequently to combine both cognitive and affective parameters in their model definition. Recently, we (Magalhães et al., 2018) employed a k-means clustering algorithm to combine results from the EPM with measurement of serum corticosterone levels. With this approach, it was possible to segregate a cohort of stressed animals into low and high responders to stress. It was thus possible to classify the responses using two completely different kinds of biomarkers. In a similar fashion, Brodnik et al. (2017) combined four behavioral measures with quantification of serum corticosterone to group responders into those that were more resilient and those that were more susceptible to depression after stress exposure. Comparison of the univariate and multivariate approaches leads to a significant strategic question regarding the design of future studies. Are univariate protocols sufficiently generalizable to yield results as robust as the multivariate? If they are proved to be so, the investigation of stress resilience and susceptibility may be made more straightforward through the use of simpler models. If this is not the case, further development of experimental designs, which incorporate more complex design and yield more generalizable models, will be required.

Potential additional biomarkers The majority of the biomarkers discussed so far are markers of behavior. Such preponderance is a direct consequence of the experimental methods used to identify stress resilience in the phenotype. As a necessary first approach, the exclusive or predominant use of such markers is associated with several important limitations and potential sources of bias. Firstly, behavioral biomarkers are inherently noisy because of the fact that the behavior observed is inevitably a composite of multiple components, which cannot be individually measured. A second issue is that, unlike the behavioral measure, an ideal biomarker, whose parameters can be objectively evaluated, should shed light on the fundamental phenomenon underlying the question of resilience or susceptibility. In the quest to reveal the actual mechanisms underlying the development of different phenotypes to stress, the development or identification of new biomarkers is required. Of the potential candidates, we will focus on one that appears to present significant potential for the minimally invasive screening of subjects. While some potential candidates (e.g., dendritic arborization Brown et al., 2005; Izquierdo et al., 2006) or epigenetic

316

20. Biomarkers of resilience and susceptibility in rodent models of stress

characterization (Bergstrom et al., 2007; Christensen et al., 2011; Elliott et al., 2010; Kenworthy et al., 2014; Stankiewicz et al., 2013) may be effective in defining the experimental subgroups, they are inherently more difficult to use routinely or rapidly and not practical to translate to clinical practice. To overcome those limitations, Chen et al. (2015) have demonstrated the potential for using the analysis of circulating microRNAs in the blood as a biomarker for identifying resilient and susceptible subjects. Alterations of the HPA axis (Tsigos and Chrousos, 2002) and an increase in the release of glucocorticoid hormones is one of the major hallmarks of stress (Sousa, 2016; Sapolsky et al., 1986). Despite this, with rare exceptions (Magalhães et al., 2018; Brodnik et al., 2017), blood corticosterone level has remained largely ignored as a biomarker of stress resilience classification but more often used to evaluate the models. One can only hypothesize on why such strategies have not yet received more attention, but the possibility exists that it is because they are indirect measures and not inherent attributes of the brain. For these reasons, we identify the continued development of imaging techniques, particularly MRI, as the most potentially valuable approach to screen for brain-wide differences, which may characterize different responders to stress. Anacker et al. (2016), through the use of both anatomical and diffusion acquisitions, were able to demonstrate differences in morphometry and fractional anisotropy, which correlated particularly well with susceptibility to social defeat stress. The findings were identified in the hippocampus, hypothalamus, bed nucleus of stria terminalis (BNST), ventral tegmental area (VTA), and cingulate cortex. Bourgin et al. (2015) have shown, using an region of interest (ROI) approach, a positive correlation between the inability to adapt to stress and amygdala volume. Using diffusion-weighted data from the same data set, we (Magalhaes et al., 2017) also demonstrated potential target areas and diffusion properties that are able to distinguish stress responders. Although these three studies do appear to represent a proof of concept for the use of MRI to screen the response to stress, they were ex vivo determinations and thus have reduced applicability. More recently, we have conducted a longitudinal study of stress susceptibility using in vivo MRI (Magalhães et al., 2018). Findings included volumetric alterations (prominent in the hippocampus, VTA, and orbital cortex) as well as a network of altered functional connectivity (centered on the thalamus and entorhinal cortex) after exposure to stress. The results suggest the technique is capable of revealing not only structural but also functional changes. Finally, and potentially most relevant for its use as a biomarker, we identified a network of altered functional connectivity in susceptible responders to stress that was present in baseline. If it is further confirmed that resting state fMRI is capable of differentiating resilient from susceptible responders before any exposure to stress; it would be a most valuable predictive biomarker. To confirm the value of the abovementioned functional connectivity network as a biomarker of susceptibility, further prospective studies that use this network as the criteria for delineating experimental groups are needed.

Conclusion The study of resilience or vulnerability to stress in rodent models is as complex as it is relevant and presents the investigator with a number of unique challenges. Any

References

317

experiments, which attempt to characterize these effects, must deal with multiple variables, only a subset of which can be controlled. Our survey of the current literature has identified several distinct approaches to the challenge. They may be functionally grouped as either prospective or retrospective in design. In the first instance, the strength of the strategy is that it is theoretically best able to test specific hypotheses and to elucidate mechanisms, but it is constrained by the requirement that a specific, preselected protocol be employed. In contrast to such an approach, studies that are of retrospective design simply seek to identify emergent correlations in the output data. For this reason, they are more suited to the discovery of unanticipated relationships between stress exposure and response than their prospective counterparts. It is most likely that some synthesis of the two approaches ultimately will be of greatest value and yield the most significant results. One potentially productive combination of strategies might be to perform serial studies, the first of retrospective design and the second prospective. From the retrospective experiment, one possible finding might be the emergence and identification of a candidate biomarker, which could subsequently be used in a prospective study to elucidate the mechanism underlying the results of the initial, retrospective analysis. In this brief review, we have concentrated on current literature, which addresses the phenomenon of resilience in rodent models of stress. It is the explicit or implicit goal of many of the studies cited to first identify, then to more fully characterize, biomarkers of the response. By this process, it is to be hoped that a portfolio of such markers will be developed. Ideally, they would differ one from another in essential character and thus be able to be used in combination to enhance both sensitivity and specificity. Following the rigorous evaluation of such an array of biomarkers in rodent and other animal models, the potential for the application of similar strategies in the clinical environment is indeed great.

Acknowledgments This work is in part financed by the Sigma project with the reference FCT-ANR/NEU-OSD/0258/2012 cofinanced by the French public funding agency ANR (Agence National pour la Recherche, APP Blanc International II 2012), the Portuguese FCT (Fundação para a Ciência e Tecnologia), and by the Portuguese North Regional Operational Program (ON.2dO Novo Norte) under the National Strategic Reference Framework (QREN), through the European Regional Development Fund (FEDER) as well as the Projeto Estratégico cofunded by FCT (PEst-C/SAU/LA0026-/2013) and the European Regional Development Fund COMPETE (FCOMP-01-0124-FEDER-037298) and the project financed by Fundação Calouste Gulbenkian (Portugal; “Better mental health during aging based on temporal prediction of individual brain aging trajectories”), Grant Number P-139977. RM is supported by the FCT fellowship grant with the reference PDE/BDE/113604/2015 from the PhD-iHES program.

References Aiello, G., Horowitz, M., Hepgul, N., Pariante, C.M., Mondelli, V., 2012. Stress abnormalities in individuals at risk for psychosis: a review of studies in subjects with familial risk or with “at risk” mental state. Psychoneuroendocrinology 37 (10), 1600e1613. Anacker, C., Scholz, J., O’Donnell, K.J., Allemang-Grand, R., Diorio, J., Bagot, R.C., et al., 2016. Neuroanatomic differences associated with stress susceptibility and resilience. Biological Psychiatry 79 (10), 840e849.

318

20. Biomarkers of resilience and susceptibility in rodent models of stress

Benatti, C., Valensisi, C., Blom, J.M., Alboni, S., Montanari, C., Ferrari, F., et al., 2012. Transcriptional profiles underlying vulnerability and resilience in rats exposed to an acute unavoidable stress. Journal of Neuroscience Research 90 (11), 2103e2115. Bergstrom, A., Jayatissa, M.N., Thykjaer, T., Wiborg, O., 2007. Molecular pathways associated with stress resilience and drug resistance in the chronic mild stress rat model of depression: a gene expression study. Journal of Molecular Neuroscience 33 (2), 201e215. Bijlsma, R., Loeschcke, V., 2005. Environmental stress, adaptation and evolution: an overview. Journal of Evolutionary Biology 18 (4), 744e749. Biomarkers Definitions Working, G., 2001. Biomarkers and surrogate endpoints: preferred definitions and conceptual framework. Clinical Pharmacology and Therapeutics 69 (3), 89e95. Bourgin, J., Cachia, A., Boumezbeur, F., Djemaï, B., Bottlaender, M., Duchesnay, E., et al., 2015. Hyper-Responsivity to Stress in Rats Is Associated with a Large Increase in Amygdala Volume. A 7T MRI Study. European Neuropsychopharmacology 25 (6), 828e835. Brodnik, Z.D., Black, E.M., Clark, M.J., Kornsey, K.N., Snyder, N.W., Espana, R.A., 2017. Susceptibility to traumatic stress sensitizes the dopaminergic response to cocaine and increases motivation for cocaine. Neuropharmacology 125, 295e307. Brown, S.M., Henning, S., Wellman, C.L., 2005. Mild, short-term stress alters dendritic morphology in rat medial prefrontal cortex. Cerebral Cortex 15 (11), 1714e1722. Castro, J.E., Diessler, S., Varea, E., Marquez, C., Larsen, M.H., Cordero, M.I., et al., 2012. Personality traits in rats predict vulnerability and resilience to developing stress-induced depression-like behaviors, HPA axis hyper-reactivity and brain changes in pERK1/2 activity. Psychoneuroendocrinology 37 (8), 1209e1223. Chen, R.J., Kelly, G., Sengupta, A., Heydendael, W., Nicholas, B., Beltrami, S., et al., 2015. MicroRNAs as biomarkers of resilience or vulnerability to stress. Neuroscience 305, 36e48. Christensen, T., Bisgaard, C.F., Wiborg, O., 2011. Biomarkers of anhedonic-like behavior, antidepressant drug refraction, and stress resilience in a rat model of depression. Neuroscience 196, 66e79. Clinton, S.M., Watson, S.J., Akil, H., 2014. High novelty-seeking rats are resilient to negative physiological effects of the early life stress. Stress 17 (1), 97e107. Cohen, H., Kozlovsky, N., Matar, M.A., Zohar, J., Kaplan, Z., 2014. Distinctive hippocampal and amygdalar cytoarchitectural changes underlie specific patterns of behavioral disruption following stress exposure in an animal model of PTSD. European Neuropsychopharmacology 24 (12), 1925e1944. de Kloet, E.R., Joels, M., Holsboer, F., 2005. Stress and the brain: from adaptation to disease. Nature Reviews Neuroscience 6 (6), 463e475. Donaldson, C., Tarrier, N., Burns, A., 1998. Determinants of carer stress in Alzheimer’s disease. International Journal of Geriatric Psychiatry 13 (4), 248e256. Elliott, E., Ezra-Nevo, G., Regev, L., Neufeld-Cohen, A., Chen, A., 2010. Resilience to social stress coincides with -functional DNA methylation of the Crf gene in adult mice. Nature Neuroscience 13 (11), 1351e1353. Faraji, J., Soltanpour, N., Lotfi, H., Moeeini, R., Moharreri, A.R., Roudaki, S., et al., 2017. Lack of social support raises stress vulnerability in rats with a history of ancestral stress. Scientific Reports 7 (1), 5277. Febbraro, F., Svenningsen, K., Tran, T.P., Wiborg, O., 2017. Neuronal substrates underlying stress resilience and susceptibility in rats. PLoS One 12 (6), e0179434. Findley, D.B., Leckman, J.F., Katsovich, L., Lin, H., Zhang, H., Grantz, H., et al., 2003. Development of the Yale Children’s Global Stress Index (YCGSI) and its application in children and adolescents ith Tourette’s syndrome and obsessive-compulsive disorder. Journal of the American Academy of Child and Adolescent Psychiatry 42 (4), 450e457. Goto, T., Kubota, Y., Toyoda, A., 2016. Effects of diet quality on vulnerability to mild subchronic social defeat stress in mice. Nutritional Neuroscience 19 (7), 284e289. Hammen, C., 2005. Stress and depression. Annual Review of Clinical Psychology 1, 293e319. Henningsen, K., Palmfeldt, J., Christiansen, S., Baiges, I., Bak, S., Jensen, O.N., et al., 2012. Candidate hippocampal biomarkers of susceptibility and resilience to stress in a rat model of depression. Molecular and Cellular Proteomics 11 (7). M111 016428. Hodes, G.E., Pfau, M.L., Leboeuf, M., Golden, S.A., Christoffel, D.J., Bregman, D., et al., 2014. Individual differences in the peripheral immune system promote resilience versus susceptibility to social stress. Proceedings of the National Academy of Sciences of the United States of America 111 (45), 16136e16141.

References

319

Hodes, G.E., Pfau, M.L., Purushothaman, I., Ahn, H.F., Golden, S.A., Christoffel, D.J., et al., 2015. Sex differences in nucleus accumbens transcriptome profiles associated with susceptibility versus resilience to subchronic variable stress. The Journal of Neuroscience 35 (50), 16362e16376. Horne, S.D., Chowdhury, S.K., Heng, H.H., 2014. Stress, genomic adaptation, and the evolutionary trade-off. Frontiers in Genetics 5, 92. Isingrini, E., Perret, L., Rainer, Q., Amilhon, B., Guma, E., Tanti, A., et al., 2016. Resilience to chronic stress is mediated by noradrenergic regulation of dopamine neurons. Nature Neuroscience 19 (4), 560e563. Issler, O., Haramati, S., Paul, E.D., Maeno, H., Navon, I., Zwang, R., et al., 2014. MicroRNA 135 is essential for chronic stress resiliency, antidepressant efficacy, and intact serotonergic activity. Neuron 83 (2), 344e360. Izquierdo, A., Wellman, C.L., Holmes, A., 2006. Brief uncontrollable stress causes dendritic retraction in infralimbic cortex and resistance to fear extinction in mice. The Journal of Neuroscience 26 (21), 5733e5738. Jackson, M., 2014. The stress of life: a modern complaint? Lancet 383 (9914), 300e301. Jochems, J., Teegarden, S.L., Chen, Y., Boulden, J., Challis, C., Ben-Dor, G.A., et al., 2015. Enhancement of stress resilience through histone deacetylase 6-mediated regulation of glucocorticoid receptor chaperone dynamics. Biological Psychiatry 77 (4), 345e355. Jones, M.P., Dilley, J.B., Drossman, D., Crowell, M.D., 2006. Brain-gut connections in functional GI disorders: anatomic and physiologic relationships. Neurogastroenterology and Motility 18 (2), 91e103. Kendler, K.S., Karkowski, L.M., Prescott, C.A., 1999. Causal relationship between stressful life events and the onset of major depression. The American Journal of Psychiatry 156 (6), 41. Kenworthy, C.A., Sengupta, A., Luz, S.M., Ver Hoeve, E.S., Meda, K., Bhatnagar, S., et al., 2014. Social defeat induces changes in histone acetylation and expression of histone modifying enzymes in the ventral hippocampus, prefrontal cortex, and dorsal raphe nucleus. Neuroscience 264, 88e98. Krishnan, V., Han, M.H., Graham, D.L., Berton, O., Renthal, W., Russo, S.J., et al., 2007. Molecular adaptations underlying susceptibility and resistance to social defeat in brain reward regions. Cell 131 (2), 391e404. Kumar, S., Hultman, R., Hughes, D., Michel, N., Katz, B.M., Dzirasa, K., 2014. Prefrontal cortex reactivity underlies trait vulnerability to chronic social defeat stress. Nature Communications 5, 4537. Ladd, C.O., Thrivikraman, K.V., Huot, R.L., Plotsky, P.M., 2005. Differential neuroendocrine responses to chronic variable stress in adult Long Evans rats exposed to handling-maternal separation as neonates. Psychoneuroendocrinology 30 (6), 520e533. Langgartner, D., Peterlik, D., Foertsch, S., Fuchsl, A.M., Brokmann, P., Flor, P.J., et al., 2017. Individual differences in stress vulnerability: the role of gut pathobionts in stress-induced colitis. Brain, Behavior, and Immunity 64, 23e32. Lebow, M., Neufeld-Cohen, A., Kuperman, Y., Tsoory, M., Gil, S., Chen, A., 2012. Susceptibility to PTSD-like behavior is mediated by corticotropin-releasing factor receptor type 2 levels in the bed nucleus of the stria terminalis. The Journal of Neuroscience 32 (20), 6906e6916. Levay, E.A., Govic, A., Hazi, A., Flannery, G., Christianson, J., Drugan, R.C., et al., 2006. Endocrine and immunological correlates of behaviorally identified swim stress resilient and vulnerable rats. Brain, Behavior, and Immunity 20 (5), 488e497. Macri, S., Granstrem, O., Shumilina, M., Antunes Gomes dos Santos, F.J., Berry, A., Saso, L., et al., 2009. Resilience and vulnerability are dose-dependently related to neonatal stressors in mice. Hormones and Behavior 56 (4), 391e398. Magalhaes, R., Bourgin, J., Boumezbeur, F., Marques, P., Bottlaender, M., Poupon, C., et al., 2017. White matter changes in microstructure associated with a maladaptive response to stress in rats. Translational Psychiatry 7 (1), e1009. Magalhães, R., Barrière, D.A., Novais, A., Marques, F., Marques, P., Cerqueira, J., Sousa, J.C., Cachia, A., Boumezbeur, F., Bottlaender, M., Jay, T.M., Mériaux, S., Sousa, N., 2018. The dynamics of stress:a longitudinal MRI study of rat brain structure and connectome. Molecular Psychiatry 23, 1998e2006. Matrov, D., Koiv, K., Kanarik, M., Peet, K., Raudkivi, K., Harro, J., 2016. Middle-range exploratory activity in adult rats suggests higher resilience to chronic social defeat. Acta Neuropsychiatrica 28 (3), 125e140. McEwen, B.S., 2003. Mood disorders and allostatic load. Biological Psychiatry 54 (3), 207. McEwen, B.S., Eiland, L., Hunter, R.G., Miller, M.M., 2012. Stress and anxiety: structural plasticity and epigenetic regulation as a consequence of stress. Neuropharmacology 62 (1), 12. McEwen, B.S., Gray, J., Nasca, C., 2015. Recognizing resilience: learning from the effects of stress on the brain. Neurobiology of Stress 1, 1e11.

320

20. Biomarkers of resilience and susceptibility in rodent models of stress

Melchior, M., Caspi, A., Milne, B.J., Danese, A., Poulton, R., Moffitt, T.E., 2007. Work stress precipitates depression and anxiety in young, working women and men. Psychological Medicine 37 (8), 1119e1129. Montes, P., Ruiz-Sanchez, E., Calvillo, M., Rojas, P., 2016. Active coping of prenatally stressed rats in the forced swimming test: involvement of the Nurr1 gene. Stress 19 (5), 506e515. Morgado, P., Freitas, D., Bessa, J.M., Sousa, N., Cerqueira, J.J., 2013. Perceived stress in obsessive-compulsive disorder is related with obsessive but not compulsive symptoms. Frontiers in Psychiatry 4, 21. Nasca, C., Bigio, B., Zelli, D., Nicoletti, F., McEwen, B.S., 2015. Mind the gap: glucocorticoids modulate hippocampal glutamate tone underlying individual differences in stress susceptibility. Molecular Psychiatry 20 (6), 755e763. O’Mahony, S.M., Marchesi, J.R., Scully, P., Codling, C., Ceolho, A.M., Quigley, E.M., et al., 2009. Early life stress alters behavior, immunity, and microbiota in rats: implications for irritable bowel syndrome and psychiatric illnesses. Biological Psychiatry 65 (3), 263e267. Parsons, P.A., 1993. Evolutionary adaptation and stress: energy budgets and habitats preferred. Behavior Genetics 23 (3), 231e238. Pearson-Leary, J., Eacret, D., Chen, R., Takano, H., Nicholas, B., Bhatnagar, S., 2017. Inflammation and vascular remodeling in the ventral hippocampus contributes to vulnerability to stress. Translational Psychiatry 7 (6), e1160. Pego, J.M., Morgado, P., Pinto, L.G., Cerqueira, J.J., Almeida, O.F., Sousa, N., 2008. Dissociation of the morphological correlates of stress-induced anxiety and fear. The European Journal of Neuroscience 27 (6), 1503e1516. Reznikov, R., Diwan, M., Nobrega, J.N., Hamani, C., 2015. Towards a better preclinical model of PTSD: characterizing animals with weak extinction, maladaptive stress responses and low plasma corticosterone. Journal of Psychiatric Research 61, 158e165. Riga, D., Schmitz, L.J.M., Hoogendijk, W.J.G., Smit, A.B., Spijker, S., 2017. Temporal profiling of depression vulnerability in a preclinical model of sustained depression. Scientific Reports 7 (1), 8570. Russo, S.J., Murrough, J.W., Han, M.H., Charney, D.S., Nestler, E.J., 2012. Neurobiology of resilience. Nature Neuroscience 15 (11), 1475e1484. Santarelli, S., Zimmermann, C., Kalideris, G., Lesuis, S.L., Arloth, J., Uribe, A., et al., 2017. An adverse early life environment can enhance stress resilience in adulthood. Psychoneuroendocrinology 78 (Suppl. C), 213e221. Sapolsky, R.M., Krey, L.C., McEwen, B.S., 1986. The neuroendocrinology of stress and aging: the glucocorticoid cascade hypothesis. Endocrine Reviews 7 (3), 284e301. Shao, W.-H., Fan, S.-H., Lei, Y., Yao, G.-E., Chen, J.-J., Zhou, J., et al., 2013. Metabolomic identification of molecular changes associated with stress resilience in the chronic mild stress rat model of depression. Metabolomics 9 (2), 433e443. Singh, A.H., Wolf, D.M., Wang, P., Arkin, A.P., 2008. Modularity of stress response evolution. Proceedings of the National Academy of Sciences of the United States of America 105 (21), 7500e7505. Sotiropoulos, I., Cerqueira, J.J., Catania, C., Takashima, A., Sousa, N., Almeida, O.F., 2008. Stress and glucocorticoid footprints in the brain-the path from depression to Alzheimer’s disease. Neuroscience and Biobehavioral Reviews 32 (6), 1161e1173. Sotiropoulos, I., Catania, C., Pinto, L.G., Silva, R., Pollerberg, G.E., Takashima, A., et al., 2011. Stress acts cumulatively to precipitate Alzheimer’s disease-like tau pathology and cognitive deficits. The Journal of Neuroscience 31 (21), 7840e7847. Sousa, N., 2016. The dynamics of the stress neuromatrix. Molecular Psychiatry 21 (3), 302e312. Southwick, S.M., Charney, D.S., 2012. The science of resilience: implications for the prevention and treatment of depression. Science 338 (6103), 79e82. Stankiewicz, A.M., Swiergiel, A.H., Lisowski, P., 2013. Epigenetics of stress adaptations in the brain. Brain Research Bulletin 98, 76e92. Stiller, A.L., Drugan, R.C., Hazi, A., Kent, S.P., 2011. Stress resilience and vulnerability: the association with rearing conditions, endocrine function, immunology, and anxious behavior. Psychoneuroendocrinology 36 (9), 1383e1395. Strimbu, K., Tavel, J.A., 2010. What are biomarkers? Current Opinion in HIV and AIDS 5 (6), 463e466. Theilmann, W., Kleimann, A., Rhein, M., Bleich, S., Frieling, H., Loscher, W., et al., 2016. Behavioral differences of male Wistar rats from different vendors in vulnerability and resilience to chronic mild stress are reflected in epigenetic regulation and expression of p11. Brain Research 1642, 505e515. Toro, J., Cervera, M., Osejo, E., Salamero, M., 1992. Obsessive-compulsive disorder in childhood and adolescence: a clinical study. The Journal of Child Psychology and Psychiatry and Allied Disciplines 33 (6), 1025e1037.

References

321

Tsigos, C., Chrousos, G.P., 2002. Hypothalamic-pituitary-adrenal axis, neuroendocrine factors and stress. Journal of Psychosomatic Research 53 (4), 865e871. Uchida, S., Nishida, A., Hara, K., Kamemoto, T., Suetsugi, M., Fujimoto, M., et al., 2008. Characterization of the vulnerability to repeated stress in Fischer 344 rats: possible involvement of microRNA-mediated down-regulation of the glucocorticoid receptor. The European Journal of Neuroscience 27 (9), 61. Uchida, S., Hara, K., Kobayashi, A., Otsuki, K., Hobara, T., Yamagata, H., et al., 2010. Maternal and genetic factors in stress-resilient and -vulnerable rats: a cross-fostering study. Brain Research 1316, 43e50. Varney, S., Polston, K.F., Jessen, T., Carneiro, A.M., 2015. Mice lacking integrin beta3 expression exhibit altered response to chronic stress. Neurobiology of Stress 2, 51e58. Walker, E., Mittal, V., Tessner, K., 2008. Stress and the hypothalamic pituitary adrenal axis in the developmental course of schizophrenia. Annual Review of Clinical Psychology 4, 189e216. Welberg, L., 2014. Psychiatric disorders: a REDD line from stress to depression. Nature Reviews Neuroscience 15 (6), 350e351. Wood, S.K., Walker, H.E., Valentino, R.J., Bhatnagar, S., 2010. Individual differences in reactivity to social stress predict susceptibility and resilience to a depressive phenotype: role of corticotropin-releasing factor. Endocrinology 151 (4), 1795e1805. Wood, S.K., Wood, C.S., Lombard, C.M., Lee, C.S., Zhang, X.Y., Finnell, J.E., et al., 2015. Inflammatory factors mediate vulnerability to a social stress-induced depressive-like phenotype in passive coping rats. Biological Psychiatry 78 (1), 38e48. Wu, G., Feder, A., Cohen, H., Kim, J.J., Calderon, S., Charney, D.S., et al., 2013. Understanding resilience. Frontiers in Behavioral Neuroscience 7, 10. Xu, L.Z., Liu, L.J., Yuan, M., Li, S.X., Yue, X.D., Lai, J.L., et al., 2016. Short photoperiod condition increases susceptibility to stress in adolescent male rats. Behavioural Brain Research 300, 38e44. Yang, C., Shirayama, Y., Zhang, J.C., Ren, Q., Hashimoto, K., 2015. Regional differences in brain-derived neurotrophic factor levels and dendritic spine density confer resilience to inescapable stress. The International Journal of Neuropsychopharmacology 18 (7), pyu121. Yang, B., Yang, C., Ren, Q., Zhang, J.C., Chen, Q.X., Shirayama, Y., et al., 2016. Regional differences in the expression of brain-derived neurotrophic factor (BDNF) pro-peptide, proBDNF and preproBDNF in the brain confer stress resilience. European Archives of Psychiatry and Clinical Neuroscience 266 (8), 765e769.

C H A P T E R

21

Maladaptive learning and the amygdaladprefrontal circuit 1

Ekaterina Likhtik1, Rony Paz2 Hunter College, The Graduate Center, City University of New York, New York, NY, United States; 2Weizmann Institute of Science, Rehovot, Israel

Modeling stress and anxiety disorders through behavioral paradigms of learning Navigation through daily life depends on a blueprint of familiar stimulusdoutcome associations and the ability to update them as circumstances change. The update is particularly important for tracking shifting sources of danger and taking immediate action for survival. Too little self-protection in the face of threat risks bodily harm, whereas indiscriminate fear is physically exhausting, psychologically debilitating, and does not promote survival. In neurobiology, the most enduring model for how we learn to associate a particular cue with an outcome is Ivan Pavlov’s model of associative learning, first formalized by Pavlov in the early 20th century (Pavlov, 1927). This model continues to be a versatile tool for studying how the nervous system learns to associate initially neutral cues with threat or safety, regulating one’s emotional state (Fanselow and Poulos, 2005). However, maladaptive learning is then associated with the dysregulated emotional state that manifests in indiscriminate anxiety, elevated autonomic tone, avoidance behaviors, and disrupted sleep, as experienced by patients suffering from posttraumatic stress disorder (PTSD) or generalized anxiety disorder (GAD) (Lissek and van Meurs, 2015; DSM-5, 2013; Jovanovic et al., 2017). The classic Pavlovian associative learning model states that associations are learned by experiencing neutral stimuli that become predictive of physically arousing unconditioned stimuli (US), and as these experiences are paired, the neutral stimuli convert to conditioned stimuli (CS) that can elicit a similar physiological response as the anticipated US. This form of paired-associate learning about threat is ubiquitous, motivating researchers to find the neural mechanisms that drive it. For a thirsty antelope in the wild, a CS may be a body of water that has previously been paired with an attacking crocodile (US), which leads to heightened autonomic arousal and increased attention, as well as cautious approach to the riverbed.

Stress Resilience https://doi.org/10.1016/B978-0-12-813983-7.00021-5

323

Copyright © 2020 Elsevier Inc. All rights reserved.

324

21. Maladaptive learning and the amygdaladprefrontal circuit

For humans, simply hearing a learned fire alarm signal (CS) that is usually paired with a fire (US) can lead to increased autonomic activation and a flight response. Traditionally, in the lab, the most often used CS is a tone or a light, and the US, a short-duration electric shock to the limbs that elicits a defensive freezing conditioned response (CR). The amount of time spent displaying defensive freezing, or “fight” response, has been the traditional readout of learning and memory for the toneeshock association. Typically, the CS and US have to meet the criteria of contingency (predictability) and contiguity (temporal proximity) for associative learning to occur. In addition, learning theories posit that new learning about a CS occurs only when there is a violation of expected outcomesdknown as a prediction errord whereby the association of the US with the CS comes as a surprise (Rescorla RA, 1972; Pearce and Bouton, 2001). Without such a discrepancy, the US is fully predicted by the CS and nothing new is learned about the CSeUS relationship (Rescorla RA, 1972; Pearce and Bouton, 2001; LePelley and McLaren, 2004; Roesch et al., 2012). In the lab, a variety of tasks are designed to simulate stimulus contingencies of the real world with tasks such as differential fear conditioning, extinction, contextual fear conditioning, avoidance training, and presentations of competing motivational stimuli (Walker and Davis, 1997; Jacobs et al., 2010; Bravo-Rivera et al., 2014; Quirk, 2006; Maren et al., 2013; Maren and Fanselow, 1997; Baxter and Murray, 2002; Sugase-Miyamoto and Richmond, 2005; Burgos-Robles et al., 2017a). The CSeUS pairings used in the lab to study aversive associations are impoverished relative to the full spectrum of stimuli and behavioral repertoires displayed by animals in the wild (Paré and Quirk, 2017; Gothard et al., 2018; Amir et al., 2015). However, they have been imminently useful for studying neural communication during behavioral states that model PTSD (Fanselow and Pennington, 2018). For instance, in these behavioral tasks, the onset and duration of the CS and US are well-defined and allow for temporally precise analysis of physiological responses, such as local field potential (LFP) oscillations and spiking activity, in structures of interest. In recent years, behavioral paradigms for the study of associative learning in animal models of anxiety have undergone expansion to account for different behavioral strategies for dealing with threat. Some paradigms have modified the US to deliver more ethologically relevant threats such as robotic predators to simulate attacks on rodents, replica snakes as a source of threat for primates, and presentation of conspecifics experiencing an aversive US as a conduit for studying learning by observing others (Shiba et al., 2017; Choi and Kim, 2010; Ito et al., 2015; Mosher et al., 2016; Allsop et al., 2018). Other tasks modulate the sensory modality of the CS to include odor or tactile stimulation (Taub et al., 2018; Lovett-Barron et al., 2014). Alternatively, the CS is modified to clearly signal the proximity of US arrival, thereby eliciting different behavioral strategies for dealing with distant versus imminent threat (e.g., avoidance vs. defensive freezing) (Fadok et al., 2017). Likewise, paradigms that provide an escape route from the US expand the study of conditioned associative learning from solely focusing on defensive freezing to more proactive strategies such as flight (Rodriguez-Romaguera et al., 2016; Moscarello and Maren, 2018; Boeke et al., 2017). Another approach to broaden the complexity of studied behavior is to present rewarding and threatening stimuli in the same context, allowing for a more complex analysis of how cue valence and competing motivations affect behavior during emotional learning (Burgos-Robles et al., 2017a; Peck and Salzman, 2014). In addition, considering the role of social hierarchy in cue encoding incorporates social structures into models of learning (Shemesh et al., 2016; Munuera et al., 2018).

Cognitive and physiological components of emotional learning

325

Nonetheless, despite the increasing complexity and better representation of behavioral choices that animals face in the wild, the core aspect of CSeUS associativity remains intact. To have an adequate representation of our surroundings, system-level mechanisms must learn to discriminate threat from nonthreat. Non-threat (CS
) refers to the part of an environment or a task, which could be a cue or a zone, learned or innate, that is not associated with danger. An important feature of the non-threatening signal is that it inhibits fearful behavioral responses. Indeed, in the case of conditioning, Pavlov described the CS
as a conditioned inhibitor of fear because it inhibited subjects’ physiological response to the CSþ (Pavlov, 1927). In keeping with this idea, the influential experimental behaviorist Robert Rescorla developed two behavioral paradigms, summation and retardation (Rescorla, 1971), for determining whether a CS is inhibitory. These paradigms test whether explicitly unpairing a CS with a US results in (1) retardation of subsequent acquisition of the conditioned emotional response to the same CS if it is later paired with aversion and (2) summation of its inhibitory properties with the excitatory properties of a CSþ, such that the conditioned emotional response to a CSþ is acquired slower if presented with the CS
(Rescorla, 1971). It should be noted that in high anxiety individuals, the safety signal can be problematic because of threat bias or increased attention and vigilance toward threat, thereby decreasing one’s ability to detect safety (Cisler and Koster, 2010; Roy et al., 2015). This deficit in disengaging from threat, and instead attending to a “feature negative” stimulus that is associated with the absence of threat, is a type of learning that could prove instrumental for decreasing pervasive anxiety.

Cognitive and physiological components of emotional learning In the case of aversive emotional learning, CSeUS associations are formed quickly, where single trial learning is sufficient to activate neuromodulator and neuroendocrine systems that contribute to long-lasting fear memory formation (Fanselow, 1990; Poulos et al., 2009; McGaugh, 2013; Diaz-Mataix et al., 2017). Indeed, such quick physiological responses sparked a debate in the early 20th century regarding the temporal order between the experienced emotion (e.g., “fear”) and the physiological responses associated with that emotion, such as changes in heart rate, respiration, and vagal tone. William James and Carl Lang proposed that physiological changes lead to the cognitive assignment of emotion, such as “fear” (JameseLang theory) (Fehr and Stern, 1970). On the contrary, Walter Cannon and Phillip Bard proposed that first feeling an emotion such as fear then leads to the associated physiological responses (CannoneBard theory) (Cannon, 1927). One model reconciling emotion assignment and physiological changes was proposed by Schachter and Singer in 1962 after conducting a study where all the participants received a shot of adrenaline, but only half of the participants were given an explanation for the physiologically arousing effects of the shot (Schachter and Singer, 1962). All of the participants were then taken to a room with either a happy or an angry person. The authors found that subjects without an explanation for their physically aroused state reported experiencing either happiness or anger depending on the emotion of the person in the room (Schachter and Singer, 1962). This work suggests that a state of high physiological arousal will be assigned a cognitive component, and when no obvious reason is perceived (e.g., no knowledge about the adrenaline shot), one’s emotional state will be labeled by using the available cues (e.g., the person in the room).

326

21. Maladaptive learning and the amygdaladprefrontal circuit

FIGURE 21.1 Physiological and cognitive responses to a threat are reciprocal. The outside blue arrows describe the direction of the physical and cognitive changes based on sensory perception of a threat. A physiological response to a perceived threat (e.g., seeing a bear in the woods while hiking), leads to a physiological response (e.g., changes in heart rate) and is coupled with a cognitive/emotional assessment (e.g., fear/vigilance). The inside red arrows show that these changes are reciprocal, whereby the cognitive assessment will enhance the physiological changes and change sensory thresholds for threat perception. Similarly, the physiological response will enhance sensory perception. The bidirectional nature of the changes is supported by the cortical/subcortical dialog. References related to this loop include: Fanselow and Pennington 2018; Allsop et al. 2018; Cisler and Koster 2010; Roy et al. 2015; Fehr and Stern 1970; Cannon 1927; Schachter and Singer, 1962; Salzman and Fusi, 2010; LeDoux and Pine 2016; Pine and LeDoux 2017; Fanselow and Pennington 2017; Resnik et al. 2011; Klavir et al. 2013; Felix-Ortiz et al. 2016; Chavez et al. 2013; Ochsner et al. 2002; Livneh and Paz 2012b; Kong et al. 2014.

In turn, the cognitive interpretation of one’s physiological state shapes the ongoing physiological response (Fig. 21.1). The question regarding a subject’s emotional state in studies using animal models continues today. Given the many components that contribute to the labeling of a particular physiological state with an emotion (e.g., intensity, valence, etc. Salzman and Fusi, 2010; Anderson and Adolphs, 2014), it has been proposed that when studying emotional learning, the focus be placed on the variety of behaviors and physiological responses to threat as one system, leaving the emotional-cognitive experience as a separate and unrelated system of study (LeDoux and Pine, 2016; Pine and LeDoux, 2017). This idea has been contested, however, by the suggestion that the removal of the cognitive-emotional appraisal and the reduction of aversive-associative learning to behavioral and physiological responses to threat diminishes the clinical relevance of the neurobiological findings for disorders such as PTSD, where physiology and cognition are highly interdependent (Pine and LeDoux, 2017; Fanselow and Pennington, 2017; Fanselow and Pennington, 2018). One positive aspect of the “two-system” approach (LeDoux and Pine, 2016) is that it allows research to focus on discovery of the neural mechanisms of learning about differently valenced associations while sidestepping the debate regarding emotions in animals. On the other hand, formulating animal models of PTSD exclusively in the context of behavioral responses to threat risks slowing down research efforts to find effective treatments in humans where the cognitive and physiological aspects are both part and parcel of the diagnosis. The intrinsically reciprocal circuitry of physiological, behavioral, and emotional responses to threat make this a looped interaction, whereby perceived threat leads to physiological changes and cognitive/emotional assessment, leading to

Associative learning in the amygdala: a preference for aversion

327

further physiological changes and changed threat perception (Resnik et al., 2011) (Fig. 21.1). As discussed below, the cognitive-emotional interplay with physiology also involves highly interconnected subcortical and cortical structures, such as the amygdala and prefrontal cortex, and makes this a difficult loop to unbuckle. While these networks are less elaborate in rodents than in humans, they nonetheless still contribute to the ongoing evaluation and adjustment of response to threat at the behavioral and cognitive level. Thus, the debate between the contribution of physiology and cognitive state to emotional learning continues, with an understanding that a careful balance of introspection about the emotional state with rigorous behavioral testing must be maintained in translational research.

Associative learning in the amygdala: a preference for aversion The amygdala, named for its almond shape by Karl Friedrich Burdach in the early 19th century, is an evolutionarily conserved structure (Fanselow and Poulos, 2005; Swanson and Petrovich, 1998). It is a collection of nuclei deep in the temporal lobe that together constitute a tightly knit microcircuit. Molecular mapping has shown that the amygdala shares its embryonic origins with several parts of the telencephalon, including the vomeronasal system, striatum, and hypothalamus (Swanson and Petrovich, 1998; Medina et al., 2011). Overall, the amygdala is widely recognized as a centralized hub for processing information about threat and for emotional associative learning (Janak and Tye, 2015; Herry and Johansen, 2014). Anatomically, it is reciprocally connected with a wide swath of subcortical structures as well as sensory cortices, receiving multiple streams of sensory input from olfactory, auditory, and visual areas (Swanson and Petrovich, 1998; McDonald, 1998; Steriade and Pare, 2007). The amygdala is also well-innervated by forebrain and midbrain neuromodulatory systems, including cholinergic, noradrenergic, serotonergic, and dopaminergic input (Steriade and Pare, 2007; Carlsen et al., 1985; Gielow and Zaborszky, 2017; Knox, 2016) and modulated by a wealth of neuropeptides, including neuropeptide S (Jungling et al., 2008), cholecystokinin (Jasnow et al., 2009), pituitary adenylate cyclaseeactivating polypeptide (Cho et al., 2012; Stevens et al., 2014), and oxytocin (Knobloch et al., 2012). The interplay of neurotransmitters and neuromodulators in the amygdala sets the stage for a complex milieu that regulates multimodal sensory integration during anxiety and threat processing. Indeed, depending on the activated receptor and cell type, the same neurotransmitter can promote or block synaptic plasticity and have opposite effects on anxiety (Li and Rainnie, 2014; Burghardt and Bauer, 2013). For example, serotonergic activation of the 5HT1A heteroreceptor in the midbrain drives an inhibitory potassium current and has anxiolytic behavioral effects, whereas serotonergic activation of the 5HT2C receptor results in increased concentrations of intracellular calcium, which leads to excitation and is associated with an anxiogenic phenotype (Burghardt and Bauer, 2013; Barnes and Sharp, 1999; Mazzone et al., 2018; Garcia-Garcia et al., 2017). Similarly, cholinergic activation stimulates both nicotinic and muscarinic receptors, which drive inhibitory and excitatory postsynaptic currents, and result in increased signal-to-noise processing as well as enhanced plasticity in the basolateral complex of the amygdala (BLA) (Pidoplichko et al., 2013; Power and Sah, 2008; Unal et al., 2015; Jiang et al., 2016; Ballinger et al., 2016). Increased cholinergic and noradrenergic transmission is

328

21. Maladaptive learning and the amygdaladprefrontal circuit

implicated in enhanced attention and memory (Tinsley et al., 2004; Power et al., 2003; Tully et al., 2007; Ballinger et al., 2016). Accordingly, forebrain and midbrain innervation of the amygdala aids in stimulus detection and encoding, as well as facilitation of plasticity at thalamic and cortical inputs to pyramidal neurons of the amygdala (McGaugh, 2013; Jiang et al., 2016; Power et al., 2003; Tully et al., 2007; Jiang et al., 2013; Uematsu et al., 2017). Furthermore, neuromodulation enables more reliable transfer of information from the amygdala to cortical regions (McGaugh, 2013; Uematsu et al., 2017; Paz and Pare, 2013). Thus, multiple transmitter systems contribute to intraamygdala processing and amygdalocortical information transfer that is critical for rapid encoding and lasting memory formation. Although traditionally the amygdala is well-known for aversive learning, recent work has demonstrated that activity in the BLA represents non-threatening and rewarding stimuli as well (Baxter and Murray, 2002; Sugase-Miyamoto and Richmond, 2005; Morrison and Salzman, 2010; Bermudez and Schultz, 2010; Peck et al., 2013; Zhang et al., 2013; Livneh and Paz, 2012a; Cole et al., 2013; Redondo et al., 2014; Lee et al., 2016; Gore et al., 2015; Burgos-Robles et al., 2017a). The BLA has been shown to encode non-threatening stimuli in a growing number of tasks and across several species (Genud-Gabai et al., 2013; Sangha et al., 2013; Senn et al., 2014; Stujenske et al., 2014; Orsini et al., 2013). For example, when animals are trained to discriminate between an aversive CSþ and a non-threatening CS
, BLA neurons become responsive to the CS
as quickly and with as much variety (increased and decreased firing) as to the CSþ (Klavir et al., 2013; Sangha et al., 2013; Sierra-Mercado et al., 2011; Grewe et al., 2017). Similarly, after the initial association has been acquired, BLA neurons continue responding to the non-threatening CS
during recall (Sangha et al., 2013; Stujenske et al., 2014; Grewe et al., 2017). Notably, a large proportion of BLA neurons that respond to the CS
have also been shown to fire to a CS-predicting reward (Sangha et al., 2013), indicating that circuits encoding the lack of threat and reward may rely on the same cell populations in the BLA, suggesting that a lack of threat may in itself be rewarding. Despite its involvement in signaling reward, when both aversive and rewarding stimuli are associatively trained, a bias develops toward representing aversive rather than positively valenced information in the BLA (Livneh and Paz, 2012a; Redondo et al., 2014; BurgosRobles et al., 2017a). In one study, a large proportion of BLA cells that fired more to a rewarding olfactory CS during training switched during recall to represent an aversive olfactory CS (Livneh and Paz, 2012a). Another study showed that once an aversive association is formed in the amygdala, the same neurons do not switch to encoding reward (Redondo et al., 2014). Similarly, stimulation of amygdala inputs to the mPFC enhances defensive freezing in the presence of an aversive CS or approach behavior in the presence of a rewarding CS, but when stimulated in the presence of both aversive and rewarding stimuli together, the amygdala drives defensive freezing (Burgos-Robles et al., 2017a). These findings are consistent with our understanding of the cellular and molecular mechanisms underlying aversive memory formation and plasticity in the amygdala, which largely come from work focusing on aversion rather than reward (Ehrlich et al., 2009; Johansen et al., 2011; Pape and Pare, 2010; Duvarci and Pare, 2014). Thus, the amygdala encodes multiple valences, but when rewarding and aversive stimuli co-occur, evidence suggests that BLA activity biases the organism toward aversion. One question of great interest has been how an amygdala neuron is recruited into a memory trace, or an engram, over others. To this end, evidence is accumulating for a key role of the transcription factor cyclic AMP/Ca2þþ response-element binding protein (CREB)

Associative learning in the medial prefrontal cortex: mixed selectivity encoding

329

(Han et al., 2009), which increases with protein synthesis during long-term potentiation and memory encoding (Kandel, 2012), for recruiting BLA neurons into an aversive association and a rewarding association (Hsiang et al., 2014). Ablating a portion of high CREBexpressing amygdala neurons during training diminishes the animals’ memory for fear conditioning at test (Yiu et al., 2014; Zhou et al., 2009). Moreover, higher CREB in BLA cells was also a mechanism for recruiting neurons into memories for rewarding stimuli, such as cocaine (Hsiang et al., 2014). As CREB upregulation is itself activity dependent (Lonze and Ginty, 2002), it’s likely that more active neurons are easier to incorporate into a new memory. BLA cells that express more CREB are slightly more depolarized, making it easier for inputs to excite them during associative learning (Stuber et al., 2011; Kandel, 2012). This raises the question whether such active, CREB-expressing cells are prewired for being incorporated into the engram by being more connected and easier to activate by afferents. Amygdala neurons that participate in encoding aversion tend to receive inputs from the ventral hippocampus and project to the mPFC and the central nucleus of the amygdala (Herry et al., 2008; Pape and Pare, 2010; Kim and Cho, 2017), whereas those encoding reward tend to project to the nucleus accumbens (Allsop et al., 2018; Stuber et al., 2011). It would be interesting to know if this connectivity pattern provides elevated spontaneous activity, driving intracellular signaling cascades and CREB expression, thereby predisposing anatomically distinct populations of projection neurons for recruitment into memory formation for aversion or reward.

Associative learning in the medial prefrontal cortex: mixed selectivity encoding The mPFC is a key cortical region for adaptive learning (Alexander and Brown, 2011), making it the ideal candidate for processing discrimination between evolving predictors of threat and non-threat (Stujenske et al., 2014; Klavir et al., 2013; Stevens et al., 2013; Stujenske et al., 2014; Motzkin et al., 2015; Karalis et al., 2016; Rozeske et al., 2018; Spellman et al., 2015; Moscarello and Maren, 2018). While many cortical regions encode disparate features of incoming stimuli, the mPFC serves as an important convergence center for cortical and subcortical inputs about various aspects of an emotionally salient stimulus. The mPFC integrates sensory signals about a cue from auditory and visual cortices (Medalla and Barbas, 2014; Sellers et al., 2015), information about stimulus valence and salience from the BLA (Felix-Ortiz et al., 2016; Klavir et al., 2017; Burgos-Robles et al., 2017b; Vogel et al., 2016; Sengupta et al., 2017), and spatial information from the hippocampus (Zelikowsky et al., 2014; Wang et al., 2016; Padilla-Coreano et al., 2016; Adhikari et al., 2011). At the same time, thalamic inputs improve communication within the mPFC (Bolkan et al., 2017; Schmitt et al., 2017) and neuromodulators such as acetylcholine contribute to cue encoding in the mPFC (Howe et al., 2017). As a result of the vast variety of processed sensory, spatial, attentional, and valence information contributing to mPFC activity during learning, it contributes to a variety of behaviors associated with emotional learning, including defensive freezing (Klavir et al., 2017; Dejean et al., 2016; Fitzgerald et al., 2015; Bukalo et al., 2015; Vollmer et al., 2016), threat avoidance (Bravo-Rivera et al., 2015), extinction, and stimulus discrimination (Giustino and Maren, 2015; Vieira et al., 2015).

330

21. Maladaptive learning and the amygdaladprefrontal circuit

Given the richness of its inputs, the mPFC is likely to use mixed selectivity encoding during threat assessment and action selection, a coding scheme that is also postulated to underlie mPFC activity during working memory and categorization tasks (Rigotti et al., 2013; Fusi et al., 2016; Kobak et al., 2016; Rigotti et al., 2013; Fusi et al., 2016; Kobak et al., 2016; Naya et al., 2017). Mixed selectivity proposes that different permutations of activity profiles across populations of cells in the mPFC select for different actions. This encoding scheme provides a multidimensional neural space for encoding anxiety-related information and allows for efficient, nonlinear, and nonesingle-feature selective encoding of highly processed input (Rigotti et al., 2013; Fusi et al., 2016; Grewe et al., 2017). In operant tasks, mPFC neurons form a variety of mixed selectivity ensembles as different actions are selected (Fig. 21.2). However, overall numbers of cells across ensembles remain similar even as ensembles change and single neurons drop in and out of the encoding population (Ma et al., 2016; Ma et al., 2014). Thus, in an actively engaged mPFC, the contribution of each neuron to the population appears to be relatively stable. It is likely that in the compromised mPFC, as in PTSD (Weber et al., 2013), there is a decreased pool of neurons readily available to fill in when some drop out of an ensemble. An important consequence of mixed selectivity encoding is action selection via downstream activation of stimulus-relevant motor output (Fusi et al., 2016; Grewe et al., 2017; Ma et al., 2016; Murugan et al., 2017). In animal models, this type of behavioral selection is tested during differential fear conditioning recall, where animals recall either an aversive CSþ or a neutral CS
, or by evaluating responses to a cue as it becomes aversive through fear conditioning and then loses its averseness via extinction training (Vieira et al., 2015). In each case, neural activity is compared between the relatively more and less aversive

FIGURE 21.2 Schematic of mixed selectivity encoding and action selection in the prefrontal cortex. As stimuli are processed, a mix of sensory, subcortical, and neuroendocrine inputs to the medial prefrontal cortex (mPFC) drive neural ensembles that incorporate multivariate information using a mixed selectivity encoding strategy. Neural ensembles in the prelimbic (PL) cortex drive inhibitory defensive responses, whereas populations in the infralimbic (IL) cortex drive excitatory responses (behavioral and autonomic). When one behavioral strategy is selected, mutual inhibition between the IL and PL inhibits competing strategies. From Itamar S Grunfeld, Ekaterina Likhtik, Mixed selectivity encoding and action selection in the prefrontal cortex during threat assessment, Current Opinion in Neurobiology, 49, 2018, 108e115.

The prelimbic and infralimbic subregions of the medial prefrontal cortex in associative learning

331

versions of the CS, which drive different action selection. Notably, the mPFC is not the only brain region that was proposed to demonstrate features of integrative or mixed selectivity encoding. For example, some hippocampal cells have been shown to encode temporal and contextual information (Sakon et al., 2014; MacDonald et al., 2011; Hsieh et al., 2014), and population activity in the amygdala has been shown to be best explained by a combination of spatial attention and stimulus valence (Grewe et al., 2017; Peck and Salzman, 2014; Peck et al., 2014). It is therefore likely that with further investigation, other regions, especially highly interconnected areas, will also demonstrate features of multidimensional feature encoding. Thus far, however, encoding in other structures was shown to have reduced dimensionality and be restricted to subsets of cells relative to the mPFC (Saez et al., 2015; Lee et al., 2016), where it is more widespread and shows higher dimensionality.

The prelimbic and infralimbic subregions of the medial prefrontal cortex in associative learning A working framework suggests that fear expression and fear suppression are supported by different subdivisions of the mPFC. The more dorsal mPFC (called the prelimbic [PL] cortex in rodents and dorsal anterior cingulate [dACC] in primates) becomes more active during fear conditioning and expression of defensive freezing (Klavir et al., 2017; Burgos-Robles et al., 2017a; Dejean et al., 2016; Fitzgerald et al., 2015; Choi et al., 2010; Sotres-Bayon et al., 2012; Courtin et al., 2014a). The ventral mPFC (called the infralimbic [IL] cortex in rodents, ventral mPFC [vmPFC] in nonhuman primates, and subgenual cortex in humans) is more associated with suppression of fear and defensive freezing (Marin et al., 2017; Fitzgerald et al., 2015; Giustino and Maren, 2015; Do-Monte et al., 2015a; An et al., 2017; An et al., 2017; Bukalo et al., 2015; Giustino and Maren, 2015; Marin et al., 2017; Bloodgood et al., 2018; Do-Monte et al., 2015b). Importantly, in addition to processing aversive stimuli, the dACC/PL is a prominent region for response adjustment in a broad range of circumstances when cues change their meaning (Bari and Robbins, 2013; Baker and Ragozzino, 2014). Accordingly, interactions of the more dorsal mPFC with the BLA are suggested to support threat-related behavior and discrimination of cues signaling threat (Sierra-Mercado et al., 2011; Klavir et al., 2013; Livneh and Paz, 2012a; Knapska et al., 2012), whereas the vmPFC is proposed to interact with the amygdala after extinction of fearful stimuli (Knapska et al., 2012; Amano et al., 2010; Bukalo et al., 2014). In a mixed selectivity model of mPFC function, neural population activity in the PL during defensive freezing or in the IL during suppression of freezing is likely to reflect the associative learning about a particular CS combined with activation of different behavioral strategies to deal with that CS, such as active escape, defensive freezing, or ignoring the cue (Fig. 21.2) (Fadok et al., 2017; Halladay and Blair, 2015; Yau and McNally, 2015). In keeping with this, unit recordings have shown that activity in a large proportion of mPFC cells predicts a particular behavioral strategy better than a single-feature identity of the CS, such that the same aversive CSþ elicits firing in some cells during freezing and in other cells during active movement (Halladay and Blair, 2015). Furthermore, pharmacological inhibition of the IL decreases active movement during the CSþ, whereas activation of the IL has the

332

21. Maladaptive learning and the amygdaladprefrontal circuit

opposite effect, suggesting that population activity in the IL is associated with selecting for behavioral strategies that involved active movement rather than defensive freezing (Halladay and Blair, 2017) whereas changes in defensive freezing as a response are controlled via projections from the PL. Thus, each subregion is likely to use mixed selectivity encoding to integrate multidimensional feature information about a CS while also activating the conditioned excitatory or inhibitory motor repertoire to that CS. These findings are in line with previous work on mixed selectivity encoding in memory tasks, which show that higher-dimension encoding, as in the mPFC, predicts behavior more accurately than lower-dimension encoding (Rigotti et al., 2013; Fusi et al., 2016), as in primary sensory cortices. Despite the notable division of labor between the PL and IL, it is not always the case that such a clean distinction can be made. There are excitatory and inhibitory connections between the PL and IL, which are likely contributing to behavior. Bipolar neuropeptide Y expressing GABAergic cells in the IL have been shown to inhibit pyramidal neurons of the PL (Saffari et al., 2016), and excitatory projections from PL layer 5/6 to the IL were shown to enhance extinction learning (Marek et al., 2018). For instance, mice with compromised extinction have increased firing rates in both the PL and IL (Fitzgerald et al., 2014), suggesting that disrupted activity in one subregion affects activity in the other. In support of this idea, mice with impaired extinction due to ethanol exposure showed anomalous physiological responses in IL neurons and morphological changes in the dendritic arbor of PL cells (Holmes et al., 2012). The interaction between these regions is an important contributor to behavioral action selection and will be clarified with further experiments examining their activity simultaneously.

Overview of amygdaladprefrontal communication during aversive emotional learning Dynamic communication between the mPFC and the BLA during behavior has emerged as a key mechanism for incorporating new information about danger into the existing blueprint. Given the fast latencies of the BLA responding to sensory stimuli (McFadyen et al., 2017), BLA input could prove to be a crucial contributor to mixed selectivity encoding and action selection in the mPFC, especially during the initial stages of acquisition (Klavir et al., 2017; Burgos-Robles et al., 2017a; Felix-Ortiz et al., 2016; Klavir et al., 2013). The amygdala is an active cortical afferent during encoding and consolidation of salient associations, when widely distributed BLA inputs are shaping plasticity at multiple cortical regions (Chavez et al., 2013; McGaugh, 2013). During behavior, BLA inputs to distal sites have been shown to play a role in shaping activity in the gustatory cortex during conditioned taste aversion (Grossman et al., 2008; Guzman-Ramos and Bermudez-Rattoni, 2012) in the cerebellum, PL, auditory cortex, and dACC during fear conditioning (Klavir et al., 2012; Chavez et al., 2013; Zhu et al., 2011) and in the rhinal cortices during appetitive conditioning (Paz and Pare, 2013). Simply pairing a tone with BLA activation, even in the absence of an aversive US, has been shown to shift the preferred frequency of neurons in the primary auditory cortex toward the frequency of the paired tone (Chavez et al., 2013). BLA projections to the PL and IL are monosynaptic and excite mPFC pyramidal cells that in turn project to the periaqueductal gray, nucleus accumbens, and also mPFC cells that project back to the amygdala (Cheriyan et al., 2016; McGarry and Carter, 2016; Senn et al., 2014). Notably, amygdala neurons that fire to the threatening CSþ have been shown to

Directionality of amygdalaeprefrontal communication during acquisition of stimulus discrimination

333

preferentially project to the PL (Senn et al., 2014; Sotres-Bayon et al., 2012), whereas those active after extinction of conditioned fear preferentially target the IL. This suggests that activity in different amygdala afferents to the mPFC during encoding could determine which mPFC subdivision is active during recall (Livneh and Paz, 2012a; Knapska et al., 2012). In addition to excitation of the mPFC, BLA input was also shown to drive strong inhibition in the mPFC. In vivo and in vitro recordings demonstrate that the BLA briefly excites pyramidal neurons but then exerts a strong inhibition (Cheriyan et al., 2016; McGarry and Carter, 2016; Dilgen et al., 2013; Arruda-Carvalho et al., 2017) via feed forward activation of parvalbumin-expressing (PVþ) and somatostatin-expressing (SOMþ) interneurons (McGarry and Carter, 2016). Simultaneous recordings in the PL and IL of fear conditioned rats show that firing rates in the IL dramatically decrease right after conditioning, while BLA input to the PL is important for production of defensive freezing (Klavir et al., 2017; BurgosRobles et al., 2017a; Sotres-Bayon et al., 2012; Giustino et al., 2016). Furthermore, BLAmediated inhibition of the IL was shown to be particularly strong onto layer 2 pyramidal cells, which project back to the BLA (McGarry and Carter, 2016). Critically, the IL-to-BLA projection is known to shut down amygdala output and decrease defensive freezing to extinguished stimuli (Fitzgerald et al., 2015; Stujenske et al., 2014; Rosenkranz and Grace, 2002). Thus, BLA activation of the mPFC could bias action selection toward defensive freezing in two ways: (1) by contributing to aversive mixed selectivity encoding in the PL (Klavir et al., 2017; Burgos-Robles et al., 2017a; Sotres-Bayon et al., 2012) while (2) simultaneously suppressing IL outputs that reduce fear, including the suppression of a reciprocal projection from the IL back to the amygdala that would suppress fear by inhibiting amygdala output (McGarry and Carter, 2016). Notably, a quiescent mPFC and overactive amygdala have been described in patients with PTSD and GAD (Stevens et al., 2017; Makovac et al., 2016), suggesting that in anxiety disorders, a hyperactive amygdala could bias for action selection of indiscriminate autonomic activation and vigilant behaviors (Sotres-Bayon et al., 2012; Klavir et al., 2017; Burgos-Robles et al., 2017a; Klavir et al., 2013; Ochsner et al., 2002).

Directionality of amygdalaeprefrontal communication during acquisition of stimulus discrimination During differential conditioning, reciprocal connectivity between the mPFC and BLA supports bidirectional information transfer between the two structures. Simultaneous cell recordings during differential conditioning show that amygdala cells that go on to differentially encode the valence associated with each CS fire before the mPFC to both CS types (Klavir et al., 2013). Indeed, BLA cells appear to have an attentional processing component, signaling new incoming associations irrespective of valence (Furlong et al., 2010; Li et al., 2011). In support of this idea, the dorsal mPFC in rats shows increased activity after trials with unexpected outcomes (Bryden et al., 2011; Furlong et al., 2010). This activity could be driven by BLA inputs that are excited during an unexpected US (Roesch et al., 2010; Li et al., 2011). Indeed, correlated mPFCeBLA firing persists throughout training on a partial reinforcement schedule, whereas it quickly diminishes during continuous reinforcement (Livneh and Paz,

334

21. Maladaptive learning and the amygdaladprefrontal circuit

2012a; Livneh and Paz, 2012b), suggesting that uncertainty or surprise related to upcoming reinforcements keeps this circuit engaged (Herry et al., 2007). As training continues and associations are learned, valence-selective BLA cells, which fire to either an aversive or a rewarding CS, follow neural firing in the mPFC (Klavir et al., 2013; Stujenske et al., 2014 ), suggesting that prefrontal gating of preferred valence in BLA cells develops with training (Fig. 21.3). Thus, the temporal development of activity in the BLAemPFC circuit could underlie the differential acquisition of stimulus valence in a changing environment. First, amygdala activation to incoming stimuli functions as an attending signal to the mPFC, where BLA-derived information is combined with other incoming input. Then, as training continues, the mPFC acquires a more multidimensional representation of a stimulus and comes to modulate amygdala activity (Klavir et al., 2013; Taub et al., 2018). Given active engagement of the mPFCeBLA circuit and temporal development of mPFCto-BLA directionality as associations are acquired, one would expect that diminished mPFCe BLA communication results in maladaptive learning. In keeping with this idea, a decrease in correlated firing in the BLAemPFC has been observed in macaques that do not successfully learn to discriminate between different CSeUS associations (Klavir et al., 2013). Likewise, simultaneously recorded BLA and mPFC LFPs show that there is higher synchrony in mice that successfully learn to discriminate between an aversive CSþ and a neutral CS
than in mice that show fear generalization (Likhtik et al., 2014; Stujenske et al., 2014). Critically, similar work in humans, assayed by resting state and functional imaging, shows that BLAemPFC connectivity and coactivation is increased when subjects discriminate between stimuli and compromised during fear generalization (Fig. 21.4) (Stevens et al., 2013; Pollak et al., 2010; Greenberg et al., 2013; Cha et al., 2014).

FIGURE 21.3 Interactions of the medial prefrontal cortex (mPFC) and basolateral complex of the amygdala (BLA) during acquisition and recall of threatening and neutral stimuli. (A) During early acquisition, the BLA leads the mPFC with faster latency responses to stimuli and contributes to stimulus encoding in the mPFC. During recall of learned information, this relationship is reversed and the mPFC leads stimulus evoked amygdala firing. (B) During early acquisition and recall of threatening stimuli, there is a synchronized theta phase reset in the mPFC and BLA. This suggests that there is a common input synchronizing both structures. (C) When neutral stimuli (such as a trained CS
) are recalled, mPFC theta leads BLA theta oscillations, BLA cells phase lock to mPFC theta, BLA fast gamma oscillations phase lock to mPFC theta oscillations, and there is fast gamma synchrony across the two structures. These relationships reflect a dialog, whereby mPFC input engages BLA interneurons either directly (via feed forward excitation) or indirectly (by driving BLA principle neurons that then excite local interneurons). See the following references for more information: Taub et al. 2018; Stujenske et al. 2014; Klavir et al. 2013; Karalis et al. 2016; Courtin et al. 2014a; Rosenkranz and Grace 2002; Likhtik et al. 2014; Lesting et al. 2011; Lesting et al. 2013; Likhtik et al. 2008; Strobel et al. 2015.

Amygdalaeprefrontal communication during recall of learned associations

335

Technique Key Electrophysiology & Behavior fMRI & Behavior Electrophysiology, Behavior & Circuit manipulation Resting state connectivity & Behavior

Unsigned error information goes from BLA-to-dACC in associative learning recall [4,18]

mPFC activation increased and amygdala decreased when fear is overcome [22]

BLA cells synchronize with mPFC theta during safety [27,16]

mPFC-to-BLA theta coherence during extinction training [31,32]

Signed error information goes from dACC-to-BLA in associative learning recall [18]

Patients with vmPFC lesions show increased BLA activation to aversive stimuli [21]

mPFC-BLA theta synchrony associated with better CS+ vs CS- discrimination [27,23]

mPFC-BLA theta synchrony in post-conditioning sleep associated with better recall [34]

High dACC-BLA theta synchrony during acquisition of a task [4,26]

Increased mPFC - BLA connectivity during discrimination [21,28]

CS-, extinction encoding in the BLA [15,16,20, 36,37]

CS- , extinction encoding in the BLA [14,19,37]

CS-, extinction encoding in the BLA [9,13]

Decreased mPFC-BLA connectivity during fear generalization [29,30] Gamma oscillations in mPFC during recall of CS[33]

Gamma oscillations in BLA & mPFC during recall of CS- [16] PV+ interneurons set prefrontal theta during fear [24]

Amygdala combines coding for context and value [17]

After extinction, mPFC inputs to BLA drive inhibition more than before [35,38,39,40]

After extinction, mPFC inputs to BLA drive inhibition more than before [25]

Amygdala encodes rewarding CS [3,10,12]

Amygdala encodes rewarding C5 [11]

Amygdala combines coding for context and value [8,5]

Amygdala encodes rewarding CS [1,2,6,7]

FIGURE 21.4 Cross-species findings in the medial prefrontal cortex (mPFC) and basolateral complex of the amygdala (BLA) circuit function during adaptive learning. Similar findings in more than one species are highlighted by boxes. The colored dots refer to the techniques used to obtain the described findings. The key techniques are found in the upper left. From Ekaterina Likhtik, Rony Paz, Amygdalaeprefrontal interactions in (mal)adaptive learning, Trends in Neurosciences, 38 (3), 2015, 158e166. Images of brains are courtesy of the University of Wisconsin and Michigan State Comparative Mammalian Brain Collections, and the National Museum of Health and Medicine (brainmuseum.org). All preparation of the specimens and images were funded by the National Science Foundation. Rat, mouse, and primate silhouettes are Wikimedia images in the public domain. Human figure reproduced with permission from P. Drubetskoy. Note: The reference numbers within the square brackets are listed in the “Suggested Reading” section at the end of the chapter.

Amygdalaeprefrontal communication during recall of learned associations Successful recall of learned associations is critical for using the established blueprint of familiar stimuluseoutcome relationships. Simultaneous recordings of neural firing and oscillatory activity in the mPFC and BLA are useful to investigate the dynamics of communication

336

21. Maladaptive learning and the amygdaladprefrontal circuit

during recall of associative learning in real time. Oscillations recorded in subcortical and cortical regions reflect waves of synchronous membrane potential fluctuations in groups of cells, shaping firing patterns and creating temporal windows when groups of neurons are receptive to input. Oscillations within a structure can synchronize with their downstream targets for long range communication (Likhtik et al., 2014; Buzsaki and Watson, 2012; Lisman and Jensen, 2013; Spellman et al., 2015). Recordings have shown that direction of information transfer in the mPFCeBLA circuit determines the successful recall of differential CSeUS associations (Stujenske et al., 2014; Klavir et al., 2013; Likhtik et al., 2014). Specifically, mPFCe BLA oscillatory activity in the theta (4e12 Hz) and high gamma (70e120 Hz) range, each considered in turn below, is associated with stimulus discrimination (Taub et al., 2018; Stujenske et al., 2014; Likhtik et al., 2014; Lesting et al., 2011; Lesting et al., 2013; Karalis et al., 2016). From rodents to humans, the power (or amplitude) of theta oscillations has been shown to increase in the mPFC and BLA with presentation of the aversive CS (Stujenske et al., 2014; Likhtik et al., 2014; Lesting et al., 2011; Mueller et al., 2014; Popa et al., 2010). Theta-range synchrony also increases between the two regions with successful recall of CS associations, and discrimination of aversive CSþ from neutral CS
(Likhtik et al., 2014) (Fig. 21.3). As described in more detail below, increased mPFCeBLA synchrony during discrimination reflects communication between these regions, with different dynamics of communication depending on subregion of the mPFC/dACC, CS valence, and schedule of reinforcement (partial or continuous). However, increased communication between the two regions is associated with discrimination between the stimuli, whereas no changes in synchrony is associated with fear generalization (Likhtik et al., 2014; Stevens et al., 2013; Pollak et al., 2010; Greenberg et al., 2013; Cha et al., 2014). During recall of differential fear conditioning, prefrontal theta oscillations entrain BLA firing only during the safe CS
, indicating that theta oscillations in the mPFC selectively organize BLA activity when fear is suppressed (Fig. 21.3). Moreover, when animals generalize fear across the CSþ and CS
, there is no increase in mPFCeBLA synchrony with stimulus presentation, and BLA firing is not entrained by prefrontal oscillations (Likhtik et al., 2014). These findings suggest that the mPFC-to-BLA projections that are active after consolidation of training (Klavir et al., 2013; Taub et al., 2018) are activated once again during recall, and that at test, theta oscillations are the likely mechanism for transferring such information between the two structures (Taub et al., 2018). At the same time, ongoing dACCeBLA communication is crucial for disambiguating the valence of a CS. Gamma oscillations are known to increase in power and synchrony during successful performance of cognitive tasks (Headley and Weinberger, 2013). Thus, it is in the mPFCeBLA circuit, where fast gamma (70e120 Hz) power and synchrony are higher during discrimination of the safe CS
(Stujenske et al., 2014). Notably, this is the opposite of a lower-range BLA gamma oscillation (30e80 Hz), which increases with fear (Courtin et al., 2014), indicating that two different neural processes underlie gamma band changes in processing safety and fear. A subset of cells in the BLA was found to be modulated by fast gamma oscillations and to fire more during diminished fear (Stujenske et al., 2014), suggesting that these cells play a role in generating local fast gamma and that they could be critical in mediating fear suppression. Given the role of inhibitory interneurons in generating gamma and pacing theta oscillations in cortex and the hippocampus (Lasztoczi and Klausberger, 2014; Beed et al., 2013), it’s likely that a set of amygdala interneurons generates the safety-related high gamma

A unified view of mPFCeBLA circuit function in adaptive learning

337

oscillation in the BLA. Moreover, inhibition in the amygdala plays a prominent role in suppressing fear-related behavior (Ehrlich et al., 2009; Trouche et al., 2013; Lee et al., 2013; Likhtik et al., 2008; Nili et al., 2010; Wolff et al., 2014; Vogel et al., 2016), and there is evidence that mPFC-to-BLA information transfer actively diminishes fear expression during recall of a CS
(Klavir et al., 2013; Likhtik et al., 2014). It’s likely that such mPFC-to-BLA communication relies on theta-frequency oscillations. BLA-high gamma power is well-modulated by mPFC theta (Stujenske et al., 2014), suggesting that mPFC inputs to the BLA feed into fast gamma generating inhibitory circuits that modulate fear suppression. In a testament to the cross-species relevance of this circuit, work in humans has demonstrated increased prefrontal gamma oscillations during presentations of the CS
(Mueller et al., 2014). Additionally, studies using fMRI show that when human subjects overcome their fear of an aversive stimulus, activity is increased in the mPFC and decreased in the amygdala (Fig. 21.4) (Nili et al., 2010). Thus, theta and multiple bands of gamma oscillations are a crucial emergent property of mPFCeBLA communication throughout recall of established aversive and nonaversive associations. Artificially reproducing these patterns via stimulation may prove to be therapeutic in PTSD patients for whom they are disrupted, and ultimately, it is important to determine the cellular subtypes and neuromodulators involved in generating this pattern of communication.

A unified view of mPFCeBLA circuit function in adaptive learning Our discussion of safe stimuli has thus far included training and recall of the neutral CS
in differential fear conditioning. However, mPFC-to-BLA directionality of information transfer and the impact of prefrontal inputs on BLA microcircuits also apply to extinction of the aversive CSþ (Sotres-Bayon and Quirk, 2010; Indovina et al., 2011; Stujenske et al., 2014; Davis et al., 2017). Increased activity in the mPFC, driven by prolonged potassium currents (Criado-Marrero et al., 2014), glutamate receptor activity (Sepulveda-Orengo et al., 2013), and BDNF release (Peters et al., 2010; Xin et al., 2014), facilitates extinction. During recall of a safe CS
, the mPFC is an important afferent that shapes postextinction inhibition in the amygdala (Amano et al., 2010; Amir et al., 2011; Cho et al., 2013) likely via prefrontal inputs feeding into inhibitory microcircuits of the amygdala (Ehrlich et al., 2009; Trouche et al., 2013; Lee et al., 2013; Likhtik et al., 2008; Cho et al., 2013; Wolff et al., 2014; Hubner et al., 2014). Notably, the majority of mPFC projections are excitatory to pyramidal neurons of the BLA (Likhtik et al., 2005; McDonald, 1998; Strobel et al., 2015), although a smaller but functional proportion activates BLA inhibitory cells (Rosenkranz and Grace, 2001; Rosenkranz and Grace, 2002). Thus, the inhibitory effects of mPFC inputs in the BLA may arise via a combination of indirect activation of interneuron populations via pyramidal cells and by feed forward activation of interneurons in the BLA (Rosenkranz and Grace, 2002; Likhtik et al., 2005; Cho et al., 2013; Strobel et al., 2015). The excitatoryeinhibitory balance at prefrontal inputs to the BLA has been shown to shift toward inhibition after extinction of conditioned fear (Amano et al., 2010; Cho et al., 2013; Strobel et al., 2015). Similarly, fast gamma oscillations increase in the mPFC and follow the same mPFC-to-BLA directionality after extinction of an aversive CSþ as during

338

21. Maladaptive learning and the amygdaladprefrontal circuit

discrimination of a safe CS
(Stujenske et al., 2014). Indeed, similar oscillatory changes and synaptic remodeling could underlie the mPFC-to-BLA signaling seen during discriminative training of a non-threatening CS like during extinction training of a CSþ (Burgos-Robles et al., 2017a; Taub et al., 2018; Rogan et al., 2005). Given the clinical advantage of boosting discrimination in patients (Jovanovic et al., 2009), it is important to identify the BLA microcircuit responsible for increasing high gamma oscillations in the BLA during non-threatening situations (Stujenske et al., 2014). In an expansion of the notion of non-threat beyond adaptive learning, data show that in a brightly lit open field (a commonly used test of innate anxiety for rodents) as animals head for the safer periphery of the enclosure, there develops an mPFC-to-BLA directionality in theta oscillations and fast gamma power increases in the BLA (Stujenske et al., 2014). In addition, as mice leave the anxiogenic center of the field and go to the periphery, BLA neurons decrease their firing, suggesting that the mPFC-to-BLA shift may be functionally shutting down at least a subset of BLA cells when aversion is decreased in innate anxiety as well as in learned fear (Stujenske et al., 2014; Amano et al., 2010; Hubner et al., 2014). Thus, the mPFC-to-BLA mode of communication during identified non-threat occurs in a range of paradigms spanning learned and innate anxiety. To identify whether the mPFCeBLA circuit dynamic described here is a true signature of “safety,” as opposed to a separate “neutral” state, future work will need to establish whether these mPFC-BLA interactions occur during explicit safety signalingdwhen a stimulus effectively identifies a period of safety (Rogan et al., 2005; Kong et al., 2014). Although this work has begun with healthy controls, it needs to be more developed in high anxiety populations where confounds such as threat bias and altered perceptual thresholds complicate safety training (Roy et al., 2015; Resnik et al., 2011; Schechtman et al., 2010). Furthermore, experiments using spatiotemporally precise manipulation of this circuit will have to demonstrate causality across multiple paradigms, which will be the basis for transferring manipulation of this circuit from animal to human work. Monitoring mPFCeBLA communication is a promising direction for developing behavioral strategies beyond extinction training, an approach that helps diminish fear in only about a third of PTSD patients and suffers from being context and stimulus dependent. However, expanding behavioral repertoires that focus on emotion regulation, discrimination of non-threat, and concomitantly develop the “pro-safety” modes of mPFCeBLA communication is a promising avenue for treating heightened fear generalization.

References Adhikari, A., Topiwala, M.A., Gordon, J.A., 2011. Single units in the medial prefrontal cortex with anxiety-related firing patterns are preferentially influenced by ventral hippocampal activity. Neuron 71 (5), 898e910. Alexander, W.H., Brown, J.W., 2011. Medial prefrontal cortex as an action-outcome predictor. Nature Neuroscience 14 (10), 1338e1344. Allsop, S.A., et al., 2018. Corticoamygdala transfer of socially derived information gates observational learning. Cell. Amano, T., Unal, C.T., Pare, D., 2010. Synaptic correlates of fear extinction in the amygdala. Nature Neuroscience 13 (4), 489e494. Amir, A., Amano, T., Pare, D., 2011. Physiological identification and infralimbic responsiveness of rat intercalated amygdala neurons. Journal of Neurophysiology 105 (6), 3054e3066. Amir, A., et al., 2015. Amygdala signaling during foraging in a hazardous environment. Journal of Neuroscience 35 (38), 12994e13005. An, B., et al., 2017. Amount of fear extinction changes its underlying mechanisms. Elife 6. Anderson, D.J., Adolphs, R., 2014. A framework for studying emotions across species. Cell 157 (1), 187e200.

References

339

Arruda-Carvalho, M., et al., 2017. Optogenetic examination of prefrontal-amygdala synaptic development. Journal of Neuroscience 37 (11), 2976e2985. Baker, P.M., Ragozzino, M.E., 2014. The prelimbic cortex and subthalamic nucleus contribute to cue-guided behavioral switching. Neurobiology of Learning and Memory 107, 65e78. Ballinger, E.C., et al., 2016. Basal forebrain cholinergic circuits and signaling in cognition and cognitive decline. Neuron 91 (6), 1199e1218. Bari, A., Robbins, T.W., 2013. Inhibition and impulsivity: behavioral and neural basis of response control. Progress in Neurobiology 108, 44e79. Barnes, N.M., Sharp, T., 1999. A review of central 5-HT receptors and their function. Neuropharmacology 38 (8), 1083e1152. Baxter, M.G., Murray, E.A., 2002. The amygdala and reward. Nature Reviews Neuroscience 3 (7), 563e573. Beed, P., et al., 2013. Inhibitory gradient along the dorsoventral axis in the medial entorhinal cortex. Neuron 79 (6), 1197e1207. Bermudez, M.A., Schultz, W., 2010. Responses of amygdala neurons to positive reward- predicting stimuli depend on background reward (contingency) rather than stimulus- reward pairing (contiguity). Journal of Neurophysiology 103 (3), 1158e1170. Bloodgood, D.W., et al., 2018. Fear extinction requires infralimbic cortex projections to the basolateral amygdala. Translational Psychiatry 8 (1), 60. Boeke, E.A., et al., 2017. Active avoidance: neural mechanisms and attenuation of pavlovian conditioned responding. Journal of Neuroscience 37 (18), 4808e4818. Bolkan, S.S., et al., 2017. Thalamic projections sustain prefrontal activity during working memory maintenance. Nature Neuroscience 20 (7), 987e996. Bravo-Rivera, C., et al., 2014. Neural structures mediating expression and extinction of platform-mediated avoidance. Journal of Neuroscience 34 (29), 9736e9742. Bravo-Rivera, C., et al., 2015. Persistent active avoidance correlates with activity in prelimbic cortex and ventral striatum. Frontiers in Behavioral Neuroscience 9, 184. Bryden, D.W., et al., 2011. Attention for learning signals in anterior cingulate cortex. Journal of Neuroscience 31 (50), 18266e18274. Bukalo, O., Pinard, C.R., Holmes, A., 2014. Mechanisms to medicines: elucidating neural and molecular substrates of fear extinction to identify novel treatments for anxiety disorders. British Journal of Pharmacology 171 (20), 4690e4718. Bukalo, D., et al., 2015. Prefrontal inputs to the amygdala instruct fear extinction memory formation. Science Advances 1 (6). Burghardt, N.S., Bauer, E.P., 2013. Acute and chronic effects of selective serotonin reuptake inhibitor treatment on fear conditioning: implications for underlying fear circuits. Neuroscience 247, 253e272. Burgos-Robles, A., et al., 2017. Amygdala inputs to prefrontal cortex guide behavior amid conflicting cues of reward and punishment. Nature Neuroscience 20 (6), 824e835. Burgos-Robles, A., et al., 2017. Amygdala inputs to prefrontal cortex guide behavior amid conflicting cues of reward and punishment. Nature Neuroscience 20 (6), 824e835. Buzsaki, G., Watson, B.O., 2012. Brain rhythms and neural syntax: implications for efficient coding of cognitive content and neuropsychiatric disease. Dialogues in Clinical Neuroscience 14 (4), 345e367. Cannon, W.B., 1927. The James-Lange theory of emotions: a critical examination and an alternative theory. American Journal of Psychology 39 (1/4), 106e124. Carlsen, J., Zaborszky, L., Heimer, L., 1985. Cholinergic projections from the basal forebrain to the basolateral amygdaloid complex: a combined retrograde fluorescent and immunohistochemical study. Journal of Comparative Neurology 234 (2), 155e167. Cha, J., et al., 2014. Circuit-wide structural and functional measures predict ventromedial prefrontal cortex fear generalization: implications for generalized anxiety disorder. Journal of Neuroscience 34 (11), 4043e4053. Chavez, C.M., McGaugh, J.L., Weinberger, N.M., 2013. Activation of the basolateral amygdala induces long-term enhancement of specific memory representations in the cerebral cortex. Neurobiology of Learning and Memory 101, 8e18. Cheriyan, J., et al., 2016. Specific targeting of the basolateral amygdala to projectionally defined pyramidal neurons in prelimbic and infralimbic cortex. eNeuro 3 (2).

340

21. Maladaptive learning and the amygdaladprefrontal circuit

Cho, J.H., et al., 2012. Pituitary adenylate cyclase-activating polypeptide induces postsynaptically expressed potentiation in the intra-amygdala circuit. Journal of Neuroscience 32 (41), 14165e14177. Cho, J.H., Deisseroth, K., Bolshakov, V.Y., 2013. Synaptic encoding of fear extinction in mPFC-amygdala circuits. Neuron 80 (6), 1491e1507. Choi, J.S., Kim, J.J., 2010. Amygdala regulates risk of predation in rats foraging in a dynamic fear environment. Proceedings of the National Academy of Sciences of the United States of America 107 (50), 21773e21777. Choi, D.C., et al., 2010. Prelimbic cortical BDNF is required for memory of learned fear but not extinction or innate fear. Proceedings of the National Academy of Sciences of the United States of America 107 (6), 2675e2680. Cisler, J.M., Koster, E.H.W., 2010. Mechanisms of attentional biases towards threat in anxiety disorders: an integrative review. Clinical Psychology Review 30 (2), 203e216. Cole, S., Powell, D.J., Petrovich, G.D., 2013. Differential recruitment of distinct amygdalar nuclei across appetitive associative learning. Learning and Memory 20 (6), 295e299. Courtin, J., et al., 2014. Persistence of amygdala gamma oscillations during extinction learning predicts spontaneous fear recovery. Neurobiology of Learning and Memory 113, 82e89. Courtin, J., et al., 2014. Prefrontal parvalbumin interneurons shape neuronal activity to drive fear expression. Nature 505 (7481), 92e96. Criado-Marrero, M., Santini, E., Porter, J.T., 2014. Modulating fear extinction memory by manipulating SK potassium channels in the infralimbic cortex. Frontiers in Behavioral Neuroscience 8, 96. Davis, P., et al., 2017. Cellular and oscillatory substrates of fear extinction learning. Nature Neuroscience 20 (11), 1624e1633. Dejean, C., et al., 2016. Prefrontal neuronal assemblies temporally control fear behaviour. Nature 535 (7612), 420e424. Diaz-Mataix, L., et al., 2017. Characterization of the amplificatory effect of norepinephrine in the acquisition of Pavlovian threat associations. Learning and Memory 24 (9), 432e439. Dilgen, J., Tejeda, H.A., O’Donnell, P., 2013. Amygdala inputs drive feedforward inhibition in the medial prefrontal cortex. Journal of Neurophysiology 110 (1), 221e229. Do-Monte, F.H., et al., 2015. Revisiting the role of infralimbic cortex in fear extinction with optogenetics. Journal of Neuroscience 35 (8), 3607e3615. Do-Monte, F.H., Quinones-Laracuente, K., Quirk, G.J., 2015. A temporal shift in the circuits mediating retrieval of fear memory. Nature 519 (7544), 460e463. DSM-5, 2013. Diagnostic and Statistical Manual of Mental Disorders, fourth ed. APA Press, Washington, D.C. Duvarci, S., Pare, D., 2014. Amygdala microcircuits controlling learned fear. Neuron 82 (5), 966e980. Ehrlich, I., et al., 2009. Amygdala inhibitory circuits and the control of fear memory. Neuron 62 (6), 757e771. Fadok, J.P., et al., 2017. A competitive inhibitory circuit for selection of active and passive fear responses. Nature 542 (7639), 96e100. Fanselow, M.S., Pennington, Z.T., 2017. The danger of LeDoux and pine’s two-system framework for fear. American Journal of Psychiatry 174 (11), 1120e1121. Fanselow, M.S., Pennington, Z.T., 2018. A return to the psychiatric dark ages with a two- system framework for fear. Behaviour Research and Therapy 100, 24e29. Fanselow, M.S., Poulos, A.M., 2005. The neuroscience of mammalian associative learning. Annual Review of Psychology 56, 207e234. Fanselow, M.S., 1990. Factors governing one-trial contextual conditioning. Animal Learning and Behavior 18 (3), 264e270. Fehr, F.S., Stern, J.A., 1970. Peripheral physiological variables and emotion: the James- Lange theory revisited. Psychological Bulletin 74 (6), 411. Felix-Ortiz, A.C., et al., 2016. Bidirectional modulation of anxiety-related and social behaviors by amygdala projections to the medial prefrontal cortex. Neuroscience 321, 197e209. Fitzgerald, P.J., et al., 2014. Prefrontal single-unit firing associated with deficient extinction in mice. Neurobiology of Learning and Memory 113, 69e81. Fitzgerald, P.J., et al., 2015. Noradrenergic blockade stabilizes prefrontal activity and enables fear extinction under stress. Proceedings of the National Academy of Sciences of the United States of America 112 (28), E3729eE3737. Furlong, T.M., et al., 2010. The role of prefrontal cortex in predictive fear learning. Behavioral Neuroscience 124 (5), 574e586. Fusi, S., Miller, E.K., Rigotti, M., 2016. Why neurons mix: high dimensionality for higher cognition. Current Opinion in Neurobiology 37, 66e74.

References

341

Garcia-Garcia, A.L., et al., 2017. Serotonin inputs to the dorsal BNST modulate anxiety in a 5- HT1A receptordependent manner. Molecular Psychiatry. Genud-Gabai, R., Klavir, O., Paz, R., 2013. Safety signals in the primate amygdala. Journal of Neuroscience 33 (46), 17986e17994. Gielow, M.R., Zaborszky, L., 2017. The input-output relationship of the cholinergic basal forebrain. Cell Reports 18 (7), 1817e1830. Giustino, T.F., Maren, S., 2015. The role of the medial prefrontal cortex in the conditioning and extinction of fear. Frontiers in Behavioral Neuroscience 9, 298. Giustino, T.F., Fitzgerald, P.J., Maren, S., 2016. Fear expression suppresses medial prefrontal cortical firing in rats. PLoS One 11 (10), e0165256. Gore, F., et al., 2015. Neural representations of unconditioned stimuli in basolateral amygdala mediate innate and learned responses. Cell 162 (1), 134e145. Gothard, K.M., et al., 2018. New perspectives on the neurophysiology of primate amygdala emerging from the study of naturalistic social behaviors. Wiley Interdisciplinary Reviews: Cognitive Science 9 (1). Greenberg, T., et al., 2013. Ventromedial prefrontal cortex reactivity is altered in generalized anxiety disorder during fear generalization. Depression and Anxiety 30 (3), 242e250. Grewe, B.F., et al., 2017. Neural ensemble dynamics underlying a long-term associative memory. Nature 543 (7647), 670e675. Grossman, S.E., et al., 2008. Learning-related plasticity of temporal coding in simultaneously recorded amygdalacortical ensembles. Journal of Neuroscience 28 (11), 2864e2873. Guzman-Ramos, K., Bermudez-Rattoni, F., 2012. Interplay of amygdala and insular cortex during and after associative taste aversion memory formation. Reviews in the Neurosciences 23 (5e6), 463e471. Halladay, L.R., Blair, H.T., 2015. Distinct ensembles of medial prefrontal cortex neurons are activated by threatening stimuli that elicit excitation vs. inhibition of movement. Journal of Neurophysiology 114 (2), 793e807. Halladay, L.R., Blair, H.T., 2017. Prefrontal infralimbic cortex mediates competition between excitation and inhibition of body movements during pavlovian fear conditioning. Journal of Neuroscience Research 95 (3), 853e862. Han, J.H., et al., 2009. Selective erasure of a fear memory. Science 323 (5920), 1492e1496. Headley, D.B., Weinberger, N.M., 2013. Fear conditioning enhances gamma oscillations and their entrainment of neurons representing the conditioned stimulus. Journal of Neuroscience 33 (13), 5705e5717. Herry, C., Johansen, J.P., 2014. Encoding of fear learning and memory in distributed neuronal circuits. Nature Neuroscience 17 (12), 1644e1654. Herry, C., et al., 2007. Processing of temporal unpredictability in human and animal amygdala. Journal of Neuroscience 27 (22), 5958e5966. Herry, C., et al., 2008. Switching on and off fear by distinct neuronal circuits. Nature 454 (7204), 600e606. Holmes, A., et al., 2012. Chronic alcohol remodels prefrontal neurons and disrupts NMDAR- mediated fear extinction encoding. Nature Neuroscience 15 (10), 1359e1361. Howe, W.M., et al., 2017. Acetylcholine release in prefrontal cortex promotes gamma oscillations and theta-gamma coupling during cue detection. Journal of Neuroscience 37 (12), 3215e3230. Hsiang, H.L., et al., 2014. Manipulating a “cocaine engram” in mice. Journal of Neuroscience 34 (42), 14115e14127. Hsieh, L.T., et al., 2014. Hippocampal activity patterns carry information about objects in temporal context. Neuron 81 (5), 1165e1178. Hubner, C., et al., 2014. Ex vivo dissection of optogenetically activated mPFC and hippocampal inputs to neurons in the basolateral amygdala: implications for fear and emotional memory. Frontiers in Behavioral Neuroscience 8, 64. Indovina, I., et al., 2011. Fear-conditioning mechanisms associated with trait vulnerability to anxiety in humans. Neuron 69 (3), 563e571. Ito, W., Erisir, A., Morozov, A., 2015. Observation of distressed conspecific as a model of emotional trauma generates silent synapses in the prefrontal-amygdala pathway and enhances fear learning, but ketamine abolishes those effects. Neuropsychopharmacology 40 (11), 2536e2545. Jacobs, N.S., Cushman, J.D., Fanselow, M.S., 2010. The accurate measurement of fear memory in Pavlovian conditioning: resolving the baseline issue. Journal of Neuroscience Methods 190 (2), 235e239. Janak, P.H., Tye, K.M., 2015. From circuits to behaviour in the amygdala. Nature 517 (7534), 284e292. Jasnow, A.M., et al., 2009. Distinct subtypes of cholecystokinin (CCK)-containing interneurons of the basolateral amygdala identified using a CCK promoter-specific lentivirus. Journal of Neurophysiology 101 (3), 1494e1506.

342

21. Maladaptive learning and the amygdaladprefrontal circuit

Jiang, L., et al., 2013. Type III neuregulin 1 is required for multiple forms of excitatory synaptic plasticity of mouse cortico-amygdala circuits. Journal of Neuroscience 33 (23), 9655e9666. Jiang, L., et al., 2016. Cholinergic signaling controls conditioned fear behaviors and enhances plasticity of corticalamygdala circuits. Neuron 90 (5), 1057e1070. Johansen, J.P., et al., 2011. Molecular mechanisms of fear learning and memory. Cell 147 (3), 509e524. Jovanovic, T., et al., 2009. Posttraumatic stress disorder may be associated with impaired fear inhibition: relation to symptom severity. Psychiatry Research 167 (1e2), 151e160. Jovanovic, T., et al., 2017. Using experimental methodologies to assess posttraumatic stress. Current Opinion Psychology 14, 23e28. Jungling, K., et al., 2008. Neuropeptide S-mediated control of fear expression and extinction: role of intercalated GABAergic neurons in the amygdala. Neuron 59 (2), 298e310. Kandel, E.R., 2012. The molecular biology of memory: cAMP, PKA, CRE, CREB-1, CREB-2, and CPEB. Molecular Brain 5, 14. Karalis, N., et al., 2016. 4-Hz oscillations synchronize prefrontal-amygdala circuits during fear behavior. Nature Neuroscience 19 (4), 605e612. Kim, W.B., Cho, J.H., 2017. Synaptic targeting of double-projecting ventral CA1 hippocampal neurons to the medial prefrontal cortex and basal amygdala. Journal of Neuroscience 37 (19), 4868e4882. Klavir, O., Genud-Gabai, R., Paz, R., 2012. Low-frequency stimulation depresses the primate anterior-cingulate-cortex and prevents spontaneous recovery of aversive memories. Journal of Neuroscience 32 (25), 8589e8597. Klavir, O., Genud-Gabai, R., Paz, R., 2013. Functional connectivity between amygdala and cingulate cortex for adaptive aversive learning. Neuron 80 (5), 1290e1300. Klavir, O., et al., 2017. Manipulating fear associations via optogenetic modulation of amygdala inputs to prefrontal cortex. Nature Neuroscience 20 (6), 836e844. Knapska, E., et al., 2012. Functional anatomy of neural circuits regulating fear and extinction. Proceedings of the National Academy of Sciences of the United States of America 109 (42), 17093e17098. Knobloch, H.S., et al., 2012. Evoked axonal oxytocin release in the central amygdala attenuates fear response. Neuron 73 (3), 553e566. Knox, D., 2016. The role of basal forebrain cholinergic neurons in fear and extinction memory. Neurobiology of Learning and Memory 133, 39e52. Kobak, D., et al., 2016. Demixed principal component analysis of neural population data. Elife 5. Kong, E., et al., 2014. Learning not to fear: neural correlates of learned safety. Neuropsychopharmacology 39 (3), 515e527. Lasztoczi, B., Klausberger, T., 2014. Layer-specific GABAergic control of distinct gamma oscillations in the CA1 hippocampus. Neuron 81 (5), 1126e1139. LeDoux, J.E., Pine, D.S., 2016. Using neuroscience to help understand fear and anxiety: a two-system framework. American Journal of Psychiatry 173 (11), 1083e1093. Lee, S., et al., 2013. Inhibitory networks of the amygdala for emotional memory. Frontiers in Neural Circuits 7, 129. Lee, S.C., et al., 2016. Basolateral amygdala nucleus responses to appetitive conditioned stimuli correlate with variations in conditioned behaviour. Nature Communications 7, 12275. LePelley, M.E., McLaren, I.P., 2004. Associative history affects the associative change undergone by both presented and absent cues in human causal learning. Journal of Experimental Psychology: Animal Behavior Processes 30 (1), 67e73. Lesting, J., et al., 2011. Patterns of coupled theta activity in amygdala-hippocampal-prefrontal cortical circuits during fear extinction. PLoS One 6 (6), e21714. Lesting, J., et al., 2013. Directional theta coherence in prefrontal cortical to amygdalo- hippocampal pathways signals fear extinction. PLoS One 8 (10), e77707. Li, C., Rainnie, D.G., 2014. Bidirectional regulation of synaptic plasticity in the basolateral amygdala induced by the D1-like family of dopamine receptors and group II metabotropic glutamate receptors. Journal of Physiology 592 (19), 4329e4351. Li, J., et al., 2011. Differential roles of human striatum and amygdala in associative learning. Nature Neuroscience 14 (10), 1250e1252. Likhtik, E., et al., 2005. Prefrontal control of the amygdala. Journal of Neuroscience 25 (32), 7429e7437.

References

343

Likhtik, E., et al., 2008. Amygdala intercalated neurons are required for expression of fear extinction. Nature 454 (7204), 642e645. Likhtik, E., et al., 2014. Prefrontal entrainment of amygdala activity signals safety in learned fear and innate anxiety. Nature Neuroscience 17 (1), 106e113. Lisman, J.E., Jensen, O., 2013. The theta-gamma neural code. Neuron 77 (6), 1002e1016. Lissek, S., van Meurs, B., 2015. Learning models of PTSD: theoretical accounts and psychobiological evidence. International Journal of Psychophysiology 98 (3 Pt 2), 594e605. Livneh, U., Paz, R., 2012. Aversive-bias and stage-selectivity in neurons of the primate amygdala during acquisition, extinction, and overnight retention. Journal of Neuroscience 32 (25), 8598e8610. Livneh, U., Paz, R., 2012. Amygdala-prefrontal synchronization underlies resistance to extinction of aversive memories. Neuron 75 (1), 133e142. Lonze, B.E., Ginty, D.D., 2002. Function and regulation of CREB family transcription factors in the nervous system. Neuron 35 (4), 605e623. Lovett-Barron, M., et al., 2014. Dendritic inhibition in the hippocampus supports fear learning. Science 343 (6173), 857e863. Ma, L., et al., 2014. Differences in the emergent coding properties of cortical and striatal ensembles. Nature Neuroscience 17 (8), 1100e1106. Ma, L., et al., 2016. A quantitative analysis of context-dependent remapping of medial frontal cortex neurons and ensembles. Journal of Neuroscience 36 (31), 8258e8272. MacDonald, C.J., et al., 2011. Hippocampal “time cells” bridge the gap in memory for discontiguous events. Neuron 71 (4), 737e749. Makovac, E., et al., 2016. Alterations in amygdala-prefrontal functional connectivity account for excessive worry and autonomic dysregulation in generalized anxiety disorder. Biological Psychiatry 80 (10), 786e795. Marek, R., et al., 2018. Excitatory connections between the prelimbic and infralimbic medial prefrontal cortex show a role for the prelimbic cortex in fear extinction. Nature Neuroscience 21 (5), 654e658. Maren, S., Fanselow, M.S., 1997. Electrolytic lesions of the fimbria/fornix, dorsal hippocampus, or entorhinal cortex produce anterograde deficits in contextual fear conditioning in rats. Neurobiology of Learning and Memory 67 (2), 142e149. Maren, S., Phan, K.L., Liberzon, I., 2013. The contextual brain: implications for fear conditioning, extinction and psychopathology. Nature Reviews Neuroscience 14 (6), 417e428. Marin, M.F., et al., 2017. Skin conductance responses and neural activations during fear conditioning and extinction recall across anxiety disorders. JAMA Psychiatry 74 (6), 622e631. Mazzone, C.M., et al., 2018. Acute engagement of Gq-mediated signaling in the bed nucleus of the stria terminalis induces anxiety-like behavior. Molecular Psychiatry 23 (1), 143e153. McDonald, A.J., 1998. Cortical pathways to the mammalian amygdala. Progress in Neurobiology 55 (3), 257e332. McFadyen, J., et al., 2017. A rapid subcortical amygdala route for faces irrespective of spatial frequency and emotion. Journal of Neuroscience 37 (14), 3864e3874. McGarry, L.M., Carter, A.G., 2016. Inhibitory gating of basolateral amygdala inputs to the prefrontal cortex. Journal of Neuroscience 36 (36), 9391e9406. McGaugh, J.L., 2013. Making lasting memories: remembering the significant. Proceedings of the National Academy of Sciences of the United States of America 110 (Suppl. 2), 10402e10407. Medalla, M., Barbas, H., 2014. Specialized prefrontal “auditory fields”: organization of primate prefrontal-temporal pathways. Frontiers in Neuroscience 8, 77. Medina, L., Bupesh, M., Abellan, A., 2011. Contribution of genoarchitecture to understanding forebrain evolution and development, with particular emphasis on the amygdala. Brain, Behavior and Evolution 78 (3), 216e236. Morrison, S.E., Salzman, C.D., 2010. Re-valuing the amygdala. Current Opinion in Neurobiology 20 (2), 221e230. Moscarello, J.M., Maren, S., 2018. Flexibility in the face of fear: hippocampal-prefrontal regulation of fear and avoidance. Current Opinion in Behavioral Sciences 19, 44e49. Mosher, C.P., et al., 2016. Tactile stimulation of the face and the production of facial expressions activate neurons in the primate amygdala. eNeuro 3 (5). Motzkin, J.C., et al., 2015. Ventromedial prefrontal cortex is critical for the regulation of amygdala activity in humans. Biological Psychiatry 77 (3), 276e284.

344

21. Maladaptive learning and the amygdaladprefrontal circuit

Mueller, E.M., et al., 2014. Prefrontal oscillations during recall of conditioned and extinguished fear in humans. Journal of Neuroscience 34 (21), 7059e7066. Munuera, J., Rigotti, M., Salzman, C.D., 2018. Shared neural coding for social hierarchy and reward value in primate amygdala. Nature Neuroscience 21 (3), 415e423. Murugan, M., et al., 2017. Combined social and spatial coding in a descending projection from the prefrontal cortex. Cell 171 (7), 1663e1677 e16. Naya, Y., et al., 2017. Contributions of primate prefrontal cortex and medial temporal lobe to temporal-order memory. Proceedings of the National Academy of Sciences of the United States of America 114 (51), 13555e13560. Nili, U., et al., 2010. Fear thou not: activity of frontal and temporal circuits in moments of real- life courage. Neuron 66 (6), 949e962. Ochsner, K.N., et al., 2002. Rethinking feelings: an FMRI study of the cognitive regulation of emotion. Journal of Cognitive Neuroscience 14 (8), 1215e1229. Orsini, C.A., Yan, C., Maren, S., 2013. Ensemble coding of context-dependent fear memory in the amygdala. Frontiers in Behavioral Neuroscience 7, 199. Padilla-Coreano, N., et al., 2016. Direct ventral hippocampal-prefrontal input is required for anxiety-related neural activity and behavior. Neuron 89 (4), 857e866. Pape, H.C., Pare, D., 2010. Plastic synaptic networks of the amygdala for the acquisition, expression, and extinction of conditioned fear. Physiological Reviews 90 (2), 419e463. Paré, D., Quirk, G.J., 2017. When scientific paradigms lead to tunnel vision: lessons from the study of fear. Npj Science of Learning 2 (1), 6. Pavlov, I.P., 1927. Conditional Reflexes: An Investigation of the Physiological Activity of the Cerebral Cortex. Paz, R., Pare, D., 2013. Physiological basis for emotional modulation of memory circuits by the amygdala. Current Opinion in Neurobiology 23 (3), 381e386. Pearce, J.M., Bouton, M.E., 2001. Theories of associative learning in animals. Annual Review of Psychology 52, 111e139. Peck, C.J., Salzman, C.D., 2014. Amygdala neural activity reflects spatial attention towards stimuli promising reward or threatening punishment. Elife 3. Peck, C.J., Lau, B., Salzman, C.D., 2013. The primate amygdala combines information about space and value. Nature Neuroscience 16 (3), 340e348. Peck, E.L., Peck, C.J., Salzman, C.D., 2014. Task-dependent spatial selectivity in the primate amygdala. Journal of Neuroscience 34 (49), 16220e16233. Peters, J., et al., 2010. Induction of fear extinction with hippocampal-infralimbic BDNF. Science 328 (5983), 1288e1290. Pidoplichko, V.I., et al., 2013. alpha7-Containing nicotinic acetylcholine receptors on interneurons of the basolateral amygdala and their role in the regulation of the network excitability. Journal of Neurophysiology 110 (10), 2358e2369. Pine, D.S., LeDoux, J.E., 2017. Elevating the role of subjective experience in the clinic: response to Fanselow and Pennington. American Journal of Psychiatry 174 (11), 1121e1122. Pollak, D.D., et al., 2010. A translational bridge between mouse and human models of learned safety. Annals of Medicine 42 (2), 115e122. Popa, D., et al., 2010. Coherent amygdalocortical theta promotes fear memory consolidation during paradoxical sleep. Proceedings of the National Academy of Sciences of the United States of America 107 (14), 6516e6519. Poulos, A.M., et al., 2009. Persistence of fear memory across time requires the basolateral amygdala complex. Proceedings of the National Academy of Sciences of the United States of America 106 (28), 11737e11741. Power, J.M., Sah, P., 2008. Competition between calcium-activated Kþ channels determines cholinergic action on firing properties of basolateral amygdala projection neurons. Journal of Neuroscience 28 (12), 3209e3220. Power, A.E., Vazdarjanova, A., McGaugh, J.L., 2003. Muscarinic cholinergic influences in memory consolidation. Neurobiology of Learning and Memory 80 (3), 178e193. Quirk, G.J., 2006. Extinction: new excitement for an old phenomenon. Biological Psychiatry 60 (4), 317e318. Redondo, R.L., et al., 2014. Bidirectional switch of the valence associated with a hippocampal contextual memory engram. Nature 513 (7518), 426e430. Rescorla, R.A.W.A., 1972. A theory of Pavlovian conditioning: variations in the effectiveness of reinforcement and nonreinforcement. In: Black AH, P.W. (Ed.), Classical Conditioning II: Current Research and Theory. Appleton Century Crofts, New York, pp. 64e99.

References

345

Rescorla, R.A., 1971. Summation and retardation tests of latent inhibition. Journal of Comparative & Physiological Psychology 75 (1), 77e81. Resnik, J., Sobel, N., Paz, R., 2011. Auditory aversive learning increases discrimination thresholds. Nature Neuroscience 14 (6), 791e796. Rigotti, M., et al., 2013. The importance of mixed selectivity in complex cognitive tasks. Nature 497 (7451), 585e590. Rodriguez-Romaguera, J., et al., 2016. An avoidance-based rodent model of exposure with response prevention therapy for obsessive-compulsive disorder. Biological Psychiatry 80 (7), 534e540. Roesch, M.R., et al., 2010. Neural correlates of variations in event processing during learning in basolateral amygdala. Journal of Neuroscience 30 (7), 2464e2471. Roesch, M.R., et al., 2012. Surprise! Neural correlates of Pearce-Hall and rescorla-wagner coexist within the brain. European Journal of Neuroscience 35 (7), 1190e1200. Rogan, M.T., et al., 2005. Distinct neural signatures for safety and danger in the amygdala and striatum of the mouse. Neuron 46 (2), 309e320. Rosenkranz, J.A., Grace, A.A., 2001. Dopamine attenuates prefrontal cortical suppression of sensory inputs to the basolateral amygdala of rats. Journal of Neuroscience 21 (11), 4090e4103. Rosenkranz, J.A., Grace, A.A., 2002. Cellular mechanisms of infralimbic and prelimbic prefrontal cortical inhibition and dopaminergic modulation of basolateral amygdala neurons in vivo. Journal of Neuroscience 22 (1), 324e337. Roy, A.K., Dennis, T.A., Warner, C.M., 2015. A critical review of attentional threat bias and its role in the treatment of pediatric anxiety disorders. Journal of Cognitive Psychotherapy 29 (3), 171e184. Rozeske, R.R., et al., 2018. Prefrontal-periaqueductal gray-projecting neurons mediate context fear discrimination. Neuron 97 (4), 898e910 e6. Saez, A., et al., 2015. Abstract context representations in primate amygdala and prefrontal cortex. Neuron 87 (4), 869e881. Saffari, R., et al., 2016. NPYþ
, but not PVþ
GABAergic neurons mediated long-range inhibition from infra- to prelimbic cortex. Translational Psychiatry 6, e736. Sakon, J.J., et al., 2014. Context-dependent incremental timing cells in the primate hippocampus. Proceedings of the National Academy of Sciences of the United States of America 111 (51), 18351e18356. Salzman, C.D., Fusi, S., 2010. Emotion, cognition, and mental state representation in amygdala and prefrontal cortex. Annual Review of Neuroscience 33, 173e202. Sangha, S., Chadick, J.Z., Janak, P.H., 2013. Safety encoding in the basal amygdala. Journal of Neuroscience 33 (9), 3744e3751. Schachter, S., Singer, J.E., 1962. Cognitive, social, and physiological determinants of emotional state. Psychological Review 69 (5), 379e399. Schechtman, E., Laufer, O., Paz, R., 2010. Negative valence widens generalization of learning. Journal of Neuroscience 30 (31), 10460e10464. Schmitt, L.I., et al., 2017. Thalamic amplification of cortical connectivity sustains attentional control. Nature 545 (7653), 219e223. Sellers, K.K., et al., 2015. Awake vs. anesthetized: layer-specific sensory processing in visual cortex and functional connectivity between cortical areas. Journal of Neurophysiology 113 (10), 3798e3815. Sengupta, A., et al., 2017. Basolateral amygdala neurons maintain aversive emotional salience. Journal of Neuroscience. Senn, V., et al., 2014. Long-range connectivity defines behavioral specificity of amygdala neurons. Neuron 81 (2), 428e437. Sepulveda-Orengo, M.T., et al., 2013. Fear extinction induces mGluR5-mediated synaptic and intrinsic plasticity in infralimbic neurons. Journal of Neuroscience 33 (17), 7184e7193. Shemesh, Y., et al., 2016. Ucn3 and CRF-R2 in the medial amygdala regulate complex social dynamics. Nature Neuroscience 19 (11), 1489e1496. Shiba, Y., et al., 2017. Converging prefronto-insula-amygdala pathways in negative emotion regulation in marmoset monkeys. Biological Psychiatry 82 (12), 895e903. Sierra-Mercado, D., Padilla-Coreano, N., Quirk, G.J., 2011. Dissociable roles of prelimbic and infralimbic cortices, ventral hippocampus, and basolateral amygdala in the expression and extinction of conditioned fear. Neuropsychopharmacology 36 (2), 529e538.

346

21. Maladaptive learning and the amygdaladprefrontal circuit

Sotres-Bayon, F., Quirk, G.J., 2010. Prefrontal control of fear: more than just extinction. Current Opinion in Neurobiology 20 (2), 231e235. Sotres-Bayon, F., et al., 2012. Gating of fear in prelimbic cortex by hippocampal and amygdala inputs. Neuron 76 (4), 804e812. Spellman, T., et al., 2015. Hippocampal-prefrontal input supports spatial encoding in working memory. Nature 522 (7556), 309e314. Steriade, M., Pare, D., 2007. The Amygdala. Gating in Cerebral Networks, pp. 54e74. Stevens, J.S., et al., 2013. Disrupted amygdala-prefrontal functional connectivity in civilian women with posttraumatic stress disorder. Journal of Psychiatric Research 47 (10), 1469e1478. Stevens, J.S., et al., 2014. PACAP receptor gene polymorphism impacts fear responses in the amygdala and hippocampus. Proceedings of the National Academy of Sciences of the United States of America 111 (8), 3158e3163. Stevens, J.S., et al., 2017. Amygdala reactivity and anterior cingulate habituation predict posttraumatic stress disorder symptom maintenance after acute civilian trauma. Biological Psychiatry 81 (12), 1023e1029. Strobel, C., et al., 2015. Prefrontal and auditory input to intercalated neurons of the amygdala. Cell Reports. Stuber, G.D., et al., 2011. Excitatory transmission from the amygdala to nucleus accumbens facilitates reward seeking. Nature 475 (7356), 377e380. Stujenske, J.M., et al., 2014. Fear and safety engage competing patterns of theta-gamma coupling in the basolateral amygdala. Neuron 83 (4), 919e933. Sugase-Miyamoto, Y., Richmond, B.J., 2005. Neuronal signals in the monkey basolateral amygdala during reward schedules. Journal of Neuroscience 25 (48), 11071e11083. Swanson, L.W., Petrovich, G.D., 1998. What is the amygdala? Trends in Neurosciences 21 (8), 323e331. Taub, A.H., et al., 2018. Oscillations synchronize amygdala-to-prefrontal primate circuits during aversive learning. Neuron 97 (2), 291e298 e3. Tinsley, M.R., Quinn, J.J., Fanselow, M.S., 2004. The role of muscarinic and nicotinic cholinergic neurotransmission in aversive conditioning: comparing pavlovian fear conditioning and inhibitory avoidance. Learning and Memory 11 (1), 35e42. Trouche, S., et al., 2013. Fear extinction causes target-specific remodeling of perisomatic inhibitory synapses. Neuron 80 (4), 1054e1065. Tully, K., et al., 2007. Norepinephrine enables the induction of associative long-term potentiation at thalamoamygdala synapses. Proceedings of the National Academy of Sciences of the United States of America 104 (35), 14146e14150. Uematsu, A., et al., 2017. Modular organization of the brainstem noradrenaline system coordinates opposing learning states. Nature Neuroscience 20 (11), 1602e1611. Unal, C.T., Pare, D., Zaborszky, L., 2015. Impact of basal forebrain cholinergic inputs on basolateral amygdala neurons. Journal of Neuroscience 35 (2), 853e863. Vieira, P.A., et al., 2015. Prefrontal NMDA receptors expressed in excitatory neurons control fear discrimination and fear extinction. Neurobiology of Learning and Memory 119, 52e62. Vogel, E., et al., 2016. Projection-specific dynamic regulation of inhibition in amygdala micro- circuits. Neuron 91 (3), 644e651. Vollmer, L.L., et al., 2016. Neuropeptide Y impairs retrieval of extinguished fear and modulates excitability of neurons in the infralimbic prefrontal cortex. Journal of Neuroscience 36 (4), 1306e1315. Walker, D.L., Davis, M., 1997. Anxiogenic effects of high illumination levels assessed with the acoustic startle response in rats. Biological Psychiatry 42 (6), 461e471. Wang, Q., Jin, J., Maren, S., 2016. Renewal of extinguished fear activates ventral hippocampal neurons projecting to the prelimbic and infralimbic cortices in rats. Neurobiology of Learning and Memory 134 (Pt A), 38e43. Weber, M., et al., 2013. Voxel-based morphometric gray matter correlates of posttraumatic stress disorder. Journal of Anxiety Disorders 27 (4), 413e419. Wolff, S.B., et al., 2014. Amygdala interneuron subtypes control fear learning through disinhibition. Nature 509 (7501), 453e458. Xin, J., et al., 2014. Involvement of BDNF signaling transmission from basolateral amygdala to infralimbic prefrontal cortex in conditioned taste aversion extinction. Journal of Neuroscience 34 (21), 7302e7313. Yau, J.O., McNally, G.P., 2015. Pharmacogenetic excitation of dorsomedial prefrontal cortex restores fear prediction error. Journal of Neuroscience 35 (1), 74e83.

Suggested Reading

347

Yiu, A.P., et al., 2014. Neurons are recruited to a memory trace based on relative neuronal excitability immediately before training. Neuron 83 (3), 722e735. Zelikowsky, M., et al., 2014. Neuronal ensembles in amygdala, hippocampus, and prefrontal cortex track differential components of contextual fear. Journal of Neuroscience 34 (25), 8462e8466. Zhang, W., et al., 2013. Functional circuits and anatomical distribution of response properties in the primate amygdala. Journal of Neuroscience 33 (2), 722e733. Zhou, Y., et al., 2009. CREB regulates excitability and the allocation of memory to subsets of neurons in the amygdala. Nature Neuroscience 12 (11), 1438e1443. Zhu, L., et al., 2011. Basolateral amygdala inactivation impairs learning-induced long-term potentiation in the cerebellar cortex. PLoS One 6 (1), e16673.

Suggested Reading [1] Baxter, M.G., Murray, E.A., 2002. The amygdala and reward. Nature Reviews Neuroscience 3 (7), 563e573. [2] Sugase-Miyamoto, Y., Richmond, B.J., 2005. Neuronal signals in the monkey basolateral amygdala during reward schedules. Journal of Neuroscience 25 (48), 11071e11083. [3] Burgos-Robles, A., et al., 2017. Amygdala inputs to prefrontal cortex guide behavior amid conflicting cues of reward and punishment. Nature Neuroscience 20 (6), 824e835. [4] Taub, A.H., et al., 2018. Oscillations synchronize amygdala-to-prefrontal primate circuits during aversive learning. Neuron 97 (2), 291e298 e3. [5] Peck, C.J., Salzman, C.D., 2014. Amygdala neural activity reflects spatial attention towards stimuli promising reward or threatening punishment. Elife 3. [6] Morrison, S.E., Salzman, C.D., 2010. Re-valuing the amygdala. Current Opinion in Neurobiology 20 (2), 221e230. [7] Bermudez, M.A., Schultz, W., 2010. Responses of amygdala neurons to positive reward- predicting stimuli depend on background reward (contingency) rather than stimulus- reward pairing (contiguity). Journal of Neurophysiology 103 (3), 1158e1170. [8] Peck, C.J., Lau, B., Salzman, C.D., 2013. The primate amygdala combines information about space and value. Nature Neuroscience 16 (3), 340e348. [9] Livneh, U., Paz, R., 2012. Aversive-bias and stage-selectivity in neurons of the primate amygdala during acquisition, extinction, and overnight retention. Journal of Neuroscience 32 (25), 8598e8610. [10] Redondo, R.L., et al., 2014. Bidirectional switch of the valence associated with a hippocampal contextual memory engram. Nature 513 (7518), 426e430. [11] Lee, S.C., et al., 2016. Basolateral amygdala nucleus responses to appetitive conditioned stimuli correlate with variations in conditioned behaviour. Nature Communications 7, 12275. [12] Gore, F., et al., 2015. Neural representations of unconditioned stimuli in basolateral amygdala mediate innate and learned responses. Cell 162 (1), 134e145. [13] Genud-Gabai, R., Klavir, O., Paz, R., 2013. Safety signals in the primate amygdala. Journal of Neuroscience 33 (46), 17986e17994. [14] Sangha, S., Chadick, J.Z., Janak, P.H., 2013. Safety encoding in the basal amygdala. Journal of Neuroscience 33 (9), 3744e3751. [15] Senn, V., et al., 2014. Long-range connectivity defines behavioral specificity of amygdala neurons. Neuron 81 (2), 428e437. [16] Stujenske, J.M., et al., 2014. Fear and safety engage competing patterns of theta-gamma coupling in the basolateral amygdala. Neuron 83 (4), 919e933. [17] Orsini, C.A., Yan, C., Maren, S., 2013. Ensemble coding of context-dependent fear memory in the amygdala. Frontiers in Behavioral Neuroscience 7, 199. [18] Klavir, O., Genud-Gabai, R., Paz, R., 2013. Functional connectivity between amygdala and cingulate cortex for adaptive aversive learning. Neuron 80 (5), 1290e1300. [19] Sierra-Mercado, D., Padilla-Coreano, N., Quirk, G.J., 2011. Dissociable roles of prelimbic and infralimbic cortices, ventral hippocampus, and basolateral amygdala in the expression and extinction of conditioned fear. Neuropsychopharmacology 36 (2), 529e538.

348

21. Maladaptive learning and the amygdaladprefrontal circuit

[20] Grewe, B.F., et al., 2017. Neural ensemble dynamics underlying a long-term associative memory. Nature 543 (7647), 670e675. [21] Stevens, J.S., et al., 2013. Disrupted amygdala-prefrontal functional connectivity in civilian women with posttraumatic stress disorder. Journal of Psychiatric Research 47 (10), 1469e1478. [22] Motzkin, J.C., et al., 2015. Ventromedial prefrontal cortex is critical for the regulation of amygdala activity in humans. Biological Psychiatry 77 (3), 276e284. [23] Karalis, N., et al., 2016. 4-Hz oscillations synchronize prefrontal-amygdala circuits during fear behavior. Nature Neuroscience 19 (4), 605e612. [24] Courtin, J., et al., 2014. Prefrontal parvalbumin interneurons shape neuronal activity to drive fear expression. Nature 505 (7481), 92e96. [25] Amano, T., Unal, C.T., Pare, D., 2010. Synaptic correlates of fear extinction in the amygdala. Nature Neuroscience 13 (4), 489e494. [26] Livneh, U., Paz, R., 2012. Amygdala-prefrontal synchronization underlies resistance to extinction of aversive memories. Neuron 75 (1), 133e142. [27] Likhtik, E., et al., 2014. Prefrontal entrainment of amygdala activity signals safety in learned fear and innate anxiety. Nature Neuroscience 17 (1), 106e113. [28] Pollak, D.D., et al., 2010. A translational bridge between mouse and human models of learned safety. Annals of Medicine 42 (2), 115e122. [29] Greenberg, T., et al., 2013. Ventromedial prefrontal cortex reactivity is altered in generalized anxiety disorder during fear generalization. Depression and Anxiety 30 (3), 242e250. [30] Cha, J., et al., 2014. Circuit-wide structural and functional measures predict ventromedial prefrontal cortex fear generalization: implications for generalized anxiety disorder. Journal of Neuroscience 34 (11), 4043e4053. [31] Lesting, J., et al., 2011. Patterns of coupled theta activity in amygdala-hippocampal-prefrontal cortical circuits during fear extinction. PLoS One 6 (6), e21714. [32] Lesting, J., et al., 2013. Directional theta coherence in prefrontal cortical to amygdalo- hippocampal pathways signals fear extinction. PLoS One 8 (10), e77707. [33] Mueller, E.M., et al., 2014. Prefrontal oscillations during recall of conditioned and extinguished fear in humans. Journal of Neuroscience 34 (21), 7059e7066. [34] Popa, D., et al., 2010. Coherent amygdalocortical theta promotes fear memory consolidation during paradoxical sleep. Proceedings of the National Academy of Sciences of the United States of America 107 (14), 6516e6519. [35] Trouche, S., et al., 2013. Fear extinction causes target-specific remodeling of perisomatic inhibitory synapses. Neuron 80 (4), 1054e1065. [36] Wolff, S.B., et al., 2014. Amygdala interneuron subtypes control fear learning through disinhibition. Nature 509 (7501), 453e458. [37] Davis, P., et al., 2017. Cellular and oscillatory substrates of fear extinction learning. Nature Neuroscience 20 (11), 1624e1633. [38] Cho, J.H., Deisseroth, K., Bolshakov, V.Y., 2013. Synaptic encoding of fear extinction in mPFC-amygdala circuits. Neuron 80 (6), 1491e1507. [39] Hubner, C., et al., 2014. Ex vivo dissection of optogenetically activated mPFC and hippocampal inputs to neurons in the basolateral amygdala: implications for fear and emotional memory. Frontiers in Behavioral Neuroscience 8, 64. [40] Strobel, C., et al., 2015. Prefrontal and auditory input to intercalated neurons of the amygdala. Cell Reports.

C H A P T E R

22

Endocannabinoid signaling and stress resilience 1

Matthew N. Hill1, Sachin Patel2

Hotchkiss Brain Institute, Cumming School of Medicine, University of Calgary, Calgary, AB, Canada; 2Departments of Psychiatry and Behavioral Sciences, Pharmacology, Molecular Physiology & Biophysics, and The Vanderbilt Brain Institute, Vanderbilt University Medical Center, Nashville, TN, United States

The endocannabinoid (eCB) system derives its name from the fact that eCBs and plantderived cannabinoids both exert their effects on physiology through common molecular receptor targets (Mechoulam and Parker, 2013). The eCB system is a neuromodulatory lipid system that consists of the cannabinoid receptor type 1 and type 2 (CB1 and CB2 receptor, respectively; Herkenham et al, 1991; Matsuda et al, 1990) and two well-studied endogenous ligands, N-arachidonoyl ethanolamine (anandamide, AEA; Devane et al, 1992) and 2-arachidonoyl glycerol (2-AG; Sugiura et al., 1995). Cannabinoid receptors were first characterized as the primary biological target of cannabis-derived tetrahydrocannabinol and couple to Gi/o proteins that function to inhibit adenylyl cyclase activity, activate potassium channels, and inhibit voltage-gated calcium channels (Howlett et al, 2002). Given that CB1 receptors are primarily localized to axon terminals, activation of these receptors results in a robust suppression of neurotransmitter release (Katona and Freund, 2012). CB1 receptors represent the most abundant class of G-proteinecoupled receptors in the central nervous system (Herkenham et al, 1991) but are also present in a variety of peripheral tissues, including the liver, adipose, vasculature, and immune cells (Howlett et al, 2002). Within the brain, CB1 receptors are primarily expressed on GABAergic and glutamatergic neurons, but they also have some expression on serotonergic, noradrenergic, and cholinergic terminals (Katona and Freund, 2012). A growing body of evidence also indicates that CB1 receptors are functionally expressed on astrocytes and can regulate the release of a host of gliotransmitters (Metna-Laurent and Marsicano, 2015; Navarette et al., 2014). CB2 receptors are mostly located in immune cells, and when activated, they can modulate immune cell migration and cytokine release both outside and within the brain (Pertwee, 2005). There is also evidence that they are possibly expressed by some neurons (Van Sickle et al, 2005), but the role of these putative neuronal

Stress Resilience https://doi.org/10.1016/B978-0-12-813983-7.00022-7

349

Copyright © 2020 Elsevier Inc. All rights reserved.

350

22. Endocannabinoid signaling and stress resilience

CB2 receptors is yet to be established (Atwood and Mackie, 2010). In addition, some eCB ligands are active at other receptor targets including peroxisome proliferatoreactivated receptor and transient receptor potential vanilloid type 1 and can also directly affect activity of some ion channels (Mechoulam and Parker, 2013). AEA and 2-AG are synthesized predominantly “on demand” from phospholipid precursors in the postsynaptic membrane by Ca2þ-dependent and -independent enzymatic processes (Katona and Freund, 2012) and feedback in a retrograde manner onto presynaptic terminals, thus suppressing afferent neurotransmitter release via activation of CB1 receptors (Katona and Freund, 2012). The synthesis of 2-AG is tightly coupled to the generation of diacylglycerol from phospholipase C activity, which is rapidly converted to 2-AG by the enzyme diacylglycerol lipase (DAGL; Sugiura et al., 1995). The synthesis of AEA, on the other hand, is far less clear and appears to be performed by at least three redundant pathways, none of which have been verified as the primary source of AEA within the brain (Ahn et al., 2008). Following release into the synaptic cleft, AEA and 2-AG are subsequently taken back into the cell by a still poorly defined uptake process mediated by a transporter mechanism (Hillard et al., 1997) and primarily degraded by distinct hydrolytic enzymes, the fatty acid amide hydrolase (FAAH; Cravatt et al., 2001) and monoacylglycerol lipase (MAGL; Dinh et al., 2002), respectively. These two degrading enzymes display distinct subcellular localization, suggesting different signaling properties for AEA and 2-AG (Gulyas et al., 2004). All studies to date have indicated that FAAH is predominantly located on intracellular membranes in postsynaptic cells, while MAGL is positioned in close proximity of CB1 receptors, in presynaptic terminals, at least within the brain regions that have been examined to date such as the hippocampus, amygdala, and cerebellum (Gulyas et al., 2004). In addition to these two primary metabolic enzymes, both AEA and 2-AG are also oxygenated by cyclooxygenase 2 to form bioactive prostaglandin derivatives (Morgan et al., 2018; Hermanson et al., 2013). Additionally, a small proportion of 2-AG is also metabolized by the alpha-beta hydrolase domain (ABHD) class of enzymes, specifically ABHD6 and ABHD12 (Blankman et al., 2007; Marrs et al., 2010). The functional role of these alternate metabolic pathways is not well characterized. For instance, because of its postsynaptic localization (Blankman et al., 2007; Marrs et al., 2010), it is possible that ABHD6 might be involved in the regulation of 2-AG levels released into the synaptic cleft. However, to date, studies have clearly identified the physiological significance of FAAH and MAGL as regulators of eCB levels as pharmacological or genetic inactivation of these two enzymes results in profound accumulation of AEA and 2-AG, respectively (Cravatt et al., 2001; Long et al., 2009). In general, eCB signaling in the synapse leads to a short or a sustained suppression of neurotransmitter release from the presynaptic compartment. Despite the fact that both AEA and 2-AG similarly act to regulate presynaptic transmitter release, it is thought that these two molecules of the eCB system may differentially contribute to phasic and tonic modes of signaling, thereby differentially mediating homeostatic, short-term, and longterm synaptic plasticity processes throughout the brain (Ahn et al., 2008; Hill and Tasker, 2012; Katona and Freund, 2012). It is thought that AEA may represent more of a “tonic” signaling molecule of the eCB system, which acts to regulate basal synaptic transmission, whereas 2-AG may represent more of a “phasic” signaling molecule activated during sustained neuronal depolarization, which in turn mediates many forms of synaptic plasticity (Ahn et al., 2008; Katona and Freund, 2012).

Impact of stress on endocannabinoid signaling

351

Anatomically, within the corticolimbic circuit that regulates the stress response, eCB synthetic and degradative enzymes and CB1 receptors are prominently expressed in the amygdala (primarily in the basolateral nucleus [BLA], but also in the central nucleus as well) (Ramikie et al., 2014; Ramikie and Patel, 2012), hippocampus, medial prefrontal cortex (mPFC), and nucleus accumbens (Herkenham et al., 1991; McPartland et al., 2007), where they modulate both excitatory and inhibitory signaling within specific neuronal circuits. This chapter will focus on how eCB signaling throughout these corticolimbic circuits modulates stress responses and describe what is known about how dynamic changes in eCB signaling in response to stress could influence the development of susceptibility or resilience to stress exposure.

Impact of stress on endocannabinoid signaling In general, most stressors have been found to have differential effects on AEA versus 2-AG signaling. The typical pattern of changes that have been documented from stress exposure find that both acute and chronic stress cause a reduction in tissue levels of AEA while also transiently elevating 2-AG levels. The magnitude of these changes seems to be somewhat amplified under conditions of repeated exposure to the same stressor (homotypic stress) and different across various regions of the brain. For a greater discussion of the nuance and details of how stress modulates eCB signaling, please refer to our previous review (Morena et al., 2016). With respect to AEA, the current model is that in response to acute stress, the stresssensitive neuropeptide cortictropin-releasing factor (CRF) is released and activates CRFR1 receptors, which then triggers the hydrolytic activity of FAAH (Gray et al., 2015; Natividad et al., 2017). This increase in FAAH activity results in a depletion of AEA levels and decline in AEA/CB1 receptor signaling. These effects are most prominently seen within the amygdala (Gray et al., 2015; Hill et al., 2009a; Patel et al., 2005; Rademacher et al., 2008), but they have also been documented to occur in the mPFC (McLaughlin et al., 2012) and the hippocampus (Wang et al., 2012; Dubreucq et al., 2012). Even 24 h following acute exposure to a footshock, brain wide levels of AEA have been found to still be reduced (Bluett et al., 2014). Similar to this, chronic exposure to stress also results in a loss of AEA levels across multiple brain regions (Hill et al., 2008a, 2010b; 2013b; Patel et al., 2005; Rademacher et al., 2008; Dubreucq et al., 2012). This effect of chronic stress again seems to be driven by a CRFR1 mechanism, but here, it appears that chronic elevations in glucocorticoid hormones drive CRF production in the amydgala and mPFC, which then results in sustained CRFR1 signaling and a consequent upregulation of AEA hydrolysis by FAAH (Bowles et al., 2012; Gray et al., 2016). Unlike AEA, stress-induced changes in 2-AG signaling go in the opposite direction, appear to be driven by different molecular mechanisms than AEA, and occur on a different time scale. Acute stress generally elevates 2-AG content in the brain (Hill et al., 2011b; Bedse et al., 2017; Wang et al., 2012; Evanson et al., 2010), although this effect does seem to require a temporal delay and is not uniformly seen after every type of stressor examined. This delay is likely driven by the fact that it does appear that elevations in glucocorticoid signaling, which occur at a later time point than CRF release does, mediate increased 2-AG signaling

352

22. Endocannabinoid signaling and stress resilience

after stress (Hill et al., 2011b; Wang et al., 2012). The effects of stress on 2-AG, similar to AEA, appear to be augmented after exposure to repeated stress, especially homotypic stress (Hill et al., 2010b; Patel et al., 2005, 2009; Dubreucq et al., 2012; Rademacher et al., 2008; Sumislawski et al., 2011). In contrast to this transient increase in 2-AG from repeated stress, several studies have recently shown a delayed reduction in 2-AG levels within limbic regions after the termination of the stress response in a manner similar to that observed for AEA (Qin et al., 2015; Hill et al., 2005; Zhong et al., 2014; Lomazzo et al., 2015).

Endocannabinoid regulation of the stress response The predominance of data generated to date indicates that a normative function of the eCB system could be to dampen or buffer against the effects of stress. Consistent with this hypothesis, pharmacological or genetic disruption of eCB signaling reliably produces a neurobehavioral phenotype that directly parallels the classical manifestation of a stress response, including activation of the hypothalamicepituitaryeadrenal (HPA) axis, increased anxiety, suppressed feeding behavior, reduced responsiveness to rewarding stimuli, hypervigilance and arousal, enhanced grooming behavior, and impaired cognitive flexibility (see Morena et al., 2016). As such, these data indicate that there is a prominent stress-inhibitory role of the eCB system. Utilizing both genetic and pharmacological approaches, it has been widely established that dynamic changes in AEA and 2-AG signaling functionally contribute to an array of physiological and behavioral changes induced by stress exposure. As AEA signaling is believed to represent a mediator of “tonic” eCB signaling, it would appear that the depletion of AEA in response to acute stress results in a disruption of tonic eCB signaling, which in turn facilitates the manifestation and orchestration of the stress response (see Morena et al., 2016 for more discussion on this topic). Consistent with this model, inhibition of AEA hydrolysis by FAAH can dampen or prevent several biological changes induced by stress exposure. For example, pharmacological augmentation of AEA signaling has been found to reduce stress-induced activation of the HPA axis (Patel et al., 2004; Hill et al., 2009a; Bedse et al., 2014; Surkin et al., 2018). The ability of FAAH inhibition to dampen activation of the HPA axis appears to largely involve the BLA as local administration of an FAAH inhibitor into the BLA dampens stress-induced HPA axis activation (Hill et al., 2009a), and local administration of a CB1 receptor antagonist into the BLA can prevent the HPA axisereducing effects of systemic FAAH inhibition (Bedse et al., 2014). Similar to effects on HPA axis activation, a loss of AEA signaling in response to stress also seems to contribute to the generation of an anxiety state. Specifically, while inhibition of FAAH was found to reduce behavioral indices of anxiety (Kathuria et al., 2003), additional work has convincingly demonstrated that FAAH inhibition is more effective at reducing anxiety-related behaviors under challenging environmental conditions or after overt stressor exposure (Bluett et al., 2014; Dincheva et al., 2015; Haller et al., 2009; Naidu et al., 2007; Hill et al., 2013b; Rossi et al., 2010; Lomazzo et al., 2015; Carnevali et al., 2015; Griebel et al., 2018; Fidelman et al., 2018; Bedse et al., 2018; Danandeh et al., 2018). Consistent with this model, we have demonstrated that stress-induced release of CRF rapidly triggers FAAH activity in the BLA to reduce AEA signaling, which in turn promotes the generation of anxiety (Gray et al., 2015; Natividad

Endocannabinoid regulation of the stress response

353

et al., 2017). Importantly, central AEA levels are negatively correlated with anxiety-like behaviors, and elevating AEA signaling can effectively curb anxiety induced by acute stress (Bluett et al., 2014; Campos et al., 2010). As such, it has been proposed that AEA may function as a mediator of “emotional homeostasis” (Marco and Viveros, 2009), functioning to keep anxiety at bay in resting conditions, from which disruption of this signal by stress could contribute to the generation of an anxious state (Gunduz-Cinar et al., 2013). In line with the ability of AEA signaling to regulate the HPA axis, it seems that the ability of AEA to regulate anxiety does involve specific actions within the BLA proper (Gray et al., 2015); however, there is also evidence to indicate that AEA signaling in the mPFC (Rubino et al., 2008) and ventral hippocampus (Campos et al., 2010) similarly gate the development of anxiety in response to stress. While stress results in a reduction in tonic AEA signaling, as discussed above, it also amplifies 2-AG signaling acutely. This increase in “phasic” eCB signaling triggered by stress exposure is believed to contribute to limiting the magnitude, and promoting the termination, of stress-induced HPA axis activity (Morena et al., 2016; Hill and Tasker, 2012). Specifically, acute administration of a CB1 receptor antagonist enhances neuronal activation within the paraventricular nucleus (PVN) in response to stress and potentiates the magnitude and duration of stress-induced corticosterone secretion (Hill et al., 2011b; Newsom et al., 2012; Patel et al., 2004; Roberts et al., 2014). Coupled to the biochemical data, these data would suggest that stress-induced elevations in 2-AG content in the mPFC and hypothalamus (Evanson et al., 2010; Hill et al., 2011b), brain regions known to be important for glucocorticoid negative feedback on the HPA axis (Dallman, 2005), contribute to termination of the stress response. Specifically, local administration of a CB1 receptor antagonist into the PVN impairs glucocorticoid-mediated rapid feedback inhibition of the HPA axis (Evanson et al., 2010), while blockade of CB1 receptors within the mPFC impairs normative recovery of the HPA axis following cessation of stress (Hill et al., 2011b). These findings are consistent with the ability of glucocorticoids to elevate 2-AG content (Atsak et al., 2012; Di et al., 2005; Hill et al, 2010a, 2011b) and suggest that 2-AG signaling is a necessary component of glucocorticoid-mediated negative feedback in the brain. With respect to anxiety, elevating 2-AG signaling through the inhibition of MAGL has been shown to limit the induction of anxiety induced by stressful, aversive environmental stimuli (Aliczki et al, 2012, 2013; Busquets-Garcia et al., 2011; Sciolino et al., 2011; Bedse et al., 2017, 2018). As stress increases 2-AG signaling, these data would suggest that the mobilization of 2-AG also acts to buffer against stress-induced anxiety and that augmentation of this signal through the inhibition of MAGL potentiates this effect. Given that stress causes a loss of tonic AEA signaling, one potential model to explain these data is that the elevations in 2-AG signaling in response to stress act to compensate for the loss of AEA signaling and provide a means of maintaining CB1 receptor signaling and preventing the induction of anxiety (Bedse et al., 2017). Inhibition of MAGL promotes 2-AG signaling and thus amplifies the ability of endogenous 2-AG to compensate for the loss of AEA, while inhibition of DAGL to deplete 2-AG and prevent this compensation results in and further exacerbation of anxiety (Bedse et al., 2017; Bluett et al., 2017). Collectively, these data present a complex picture of how eCB signaling regulates the stress response. Exposure to stress results in a rapid induction of CRF signaling, which triggers FAAH activity and depletes tonic AEA signaling. This loss of AEA/CB1 receptor signaling

354

22. Endocannabinoid signaling and stress resilience

contributes to the activation of the stress response, which then promotes the release of glucocorticoid hormones. Once glucocorticoids enter the brain, they enhance 2-AG signaling, which then compensates for the loss of AEA signaling at CB1 receptors and acts to dampen neuronal activation in stress-responsive circuits, such as the BLA, mPFC, and PVN, which then facilitates termination of the stress response. For a more in-depth discussion regarding how eCB signaling regulates acute stress-induced activation of the HPA axis and behavioral changes produced by stress, please refer to Hill and Tasker (2012) and Morena et al. (2016).

Endocannabinoid signaling in the context of susceptibility and resilience to repeated stress Acute stress has proven to be a useful model to understand the mechanisms and dynamics by which eCB signaling can modulate stress-related outcomes, but with respect to disease, outside of exposure to extreme, traumatic stress (such as the case is for posttraumatic stress disorder; PTSD), the relationship of stress to pathology typically occurs in the context of repeated or chronic stress (McEwen and Gianaros, 2011). Here, individual differences in the magnitude and nature of responses to repeated stress tend to associate with susceptibility to the development stress-related psychiatric illnesses, such as mood and anxiety disorders, or resilience to the onset of these disease states (McEwen and Gianaros, 2011; Karatsoreos and McEwen, 2011). Clearly, understanding the neural mechanisms subserving susceptibility or resilience to stress are paramount to targeting novel approaches to the treatment of stressrelated psychiatric illnesses. At a most basic level, one perspective on susceptibility and resilience is that a failure to engage in typical, adaptive responses to repeated/chronic stress enhances susceptibility of an organism to the adverse effects of stress. A clear example of this is an inability of the HPA axis to appropriately adapt to repeated stress and how excess activation of this axis, and persistent secretion of glucocorticoid hormones, can result in biological changes that relate to the development of mood and anxiety disorders (McEwen and Gianaros, 2011; Karatsoreos and McEwen, 2011). As the study of susceptibility and resilience has increased in recent years (Russo et al., 2012), another perspective has developed, which suggests that the development of resilience to stress relates to individual differences in the engagement of active neurobiological processes that favor resilience by constraining the adverse effects of stress (Russo et al., 2012). Although the current state of the literature exploring the role of eCB signaling in stress resilience is relatively sparse, several key findings do support a potentially important role of eCB signaling in this process. The importance of eCB signaling in mitigating the adverse effects of chronic stress exposure has been well described. As mentioned, adaptation of the HPA axis to repeated stress (a process referred to as “stress habituation”; Grissom and Bhatnagar, 2009) is considered a beneficial response as it limits the exposure of an organism to persistently elevated levels of this catabolic hormone, which is known to promote structural and metabolic alterations in the brain (Karatsoreos and McEwen, 2011). The role of eCB signaling in the process of stress adaptation has been well established. First off, mice lacking CB1 receptors have been found to exhibit impaired habituation of behavioral responses to repeated exposure to audiogenic stress (Fride et al., 2005). Similarly, the impacts of chronic stress on various end points

Endocannabinoid signaling in the context of susceptibility and resilience to repeated stress

355

of emotional behavior related to anxiety or reward sensitivity have all been shown to be exacerbated in mice lacking CB1 receptors (Hill et al., 2011a; Martin et al., 2002; Dubreucq et al., 2012). Given the potential confounds associated with global and germline deletion of the CB1 receptor, it is important to highlight pharmacological studies that have more convincingly demonstrated the importance of eCB signaling to stress adaptation and habituation. Acute pharmacological blockade of CB1 receptors in animals that have been repeatedly exposed to stressors has found what is best described as a “dishabituation” of the stress response. First demonstrated by Patel et al. (2005), animals repeatedly exposed to restraint stress exhibit habituation of both stress-induced behavioral struggling and the induction of the activity-dependent early immediate gene c-fos throughout stress regulatory brain regions. Administration of a CB1 receptor antagonist immediately before exposure to the final stressor resulted in a reversal of habituation characterized by elevations in struggling behavior and c-fos induction comparable with that seen under acute, novel conditions. These data would indicate that, in a stress-dependent manner, 2-AG levels progressively increase in the brain to reduce neuronal activation in stress-responsive circuits and promote habituation of the neurobehavioral responses to stress. Consistent with this, additional work from Hill et al. (2010b) focused on the amygdala specifically and found that repeated stress produced a robust and transient increase in 2-AG content within the amygdala and that local blockade of CB1 receptors just within the BLA itself was sufficient to reverse habituation of the HPA axis. These data have led to the hypothetical model that eCB signaling is essential for stress adaptation and that the active recruitment of 2-AG signaling during repeated stress dampens neural circuits activated by stress, thereby resulting in a system-level habituation to stress (Hill et al., 2010b; Patel and Hillard, 2008). Given the importance of 2-AG signaling during repeated stress to promote normative stress adaptation, it is logical to assume that eCB signaling in turn would relate to the development of resilience. The first evidence to functionally demonstrate this came from a study performed by Bluett et al. (2017). In this study, mice were repeatedly exposed to footshock stress and segregated based on their response to develop anxiety in response to shock (susceptible) or show no changes in anxiety-like behavior, following shock relative to their baseline behavior (resilient). This effect was stable and replicable and maintained after multiple exposures to stress. Pharmacological blockade of CB1 receptors elevated anxiety under basal conditions and amplified stress-induced anxiety in this model similar as to what has been found in previous reports. Importantly, when looking at the proportion of animals that would be categorized as “susceptible” or “resilient” after exposure to repeated stress, depletion of 2-AG through pharmacological disruption of DAGL activity roughly tripled the percentage of animals that were “susceptible,” resulting in a significant reduction in the proportion of animals that were “resilient”. In line with this, and with the previous work highlighting the BLA as an important nexus for the stress-inhibitory role of 2-AG signaling, genetic deletion of DAGL exclusively within the BLA similarly resulted in an approximate 50% reduction in the proportion of animals that were classified as “resilient” following repeated stress. Taken together, these data all support a model by which the progressive recruitment of 2-AG signaling in response to repeated stress is required for the development of stress resilience.

356

22. Endocannabinoid signaling and stress resilience

In parallel to these studies, pharmacological augmentation of 2-AG signaling has similarly been found to promote the development of stress resilience. Further work from Bluett et al. (2017) found that while depleting 2-AG favored the development of susceptibility to stress, elevating 2-AG signaling through the blockade of MAGL elevated the proportion of animals that were “resilient” following exposure to repeated stress. In fact, following treatment with an MAGL inhibitor, it was found that less than 10% of animals were classified as “susceptible” following repeated stress exposure, as opposed to the 25%e35% that were typically found to be “susceptible.” Building off of these findings, further work found that these changes in resilience and susceptibility related to eCB-mediated regulation of synaptic transmission within the BLA. Specifically, animals that were found to be resilient after stress were also found to have relatively enhanced eCB-mediated retrograde inhibition BLA glutamatergic synaptic transmission than mice that were susceptible to stress. The administration of an MAGL inhibitor was found to reduce glutamatergic transmission in the BLA in both resilient and susceptible animals, indicating that pharmacological interventions that elevated 2-AG were sufficient to reduce excitatory synaptic transmission in the BLA, which is consistent with the ability of MAGL inhibition to promote stress resilience. Further circuit-based work using optogenetics demonstrated that the difference between resilient and susceptible animals could be linked to differences in the ability of 2-AG signaling to regulate excitatory input to the BLA from the ventral hippocampus. As such, this work indicates that in responses to repeated stress exposure, animals that recruit 2-AG signaling within the BLA in response to repeated stress, and thus exhibit a greater dynamic range in their ability to regulate afferent excitatory input to the BLA, are those which exhibit stress resilience, while those that do not mount this 2-AG responses favor the development of stress susceptibility. In line with these findings, similar work from Bosch-Bouju et al. (2016) also found that following exposure to repeated stress, animals could be categorized as resilient or susceptible based on their development of anxiety. Again, the development of resilience was related to changes in synaptic function, albeit in this study, the focus was on the nucleus accumbens where spike timingedependent plasticity was abolished in animals that developed susceptibility. Consistent with the findings described above, administration of an MAGL inhibitor favored the development of resilience over susceptibility in tandem with its ability to normalize eCB-mediated plasticity changes. Although these findings do implicate additional brain regions beyond the BLA, it is striking to note the parallels in the data set and the consistency between these studies in the ability of MAGL inhibition and the elevation of 2-AG signaling to promote resilience in the face of repeated stress. All of the discussion on resilience to this point has focused on 2-AG, as this is where most of the data are found. It is important to note, however, that inhibition of FAAH and elevations in AEA signaling have also been found to counter the effects of chronic stress on a multitude of end points including reward sensitivity, anxiety, and both synaptic and structural plasticity (Hill et al., 2013b; Rossi et al., 2010; Lomazzo et al., 2015; Griebel et al., 2018; Danandeh et al., 2018). While FAAH inhibition has never been examined in explicit studies looking at the development of stress susceptibility or resilience, based on what we know about eCB signaling, and the ability of elevated AEA signaling to counter the effects of impaired 2-AG signaling, it seems plausible to hypothesize that elevating AEA signaling may also favor stress resilience. Further work is required to determine the role of AEA signaling in the development of stress susceptibility versus resilience.

357

Conclusions

Conclusions

Resilience

STRESS

STRESS

STRESS

Vulnerability

Endocannabinoid Activity

This chapter reviewed the data regarding what is known about interactions between stress and the eCB system and how eCB signaling could influence the development of stress resilience after exposure to repeated stress. The model that can be generated from the current state of the science would suggest that stress resilience is driven, at least in part, by a progressive recruitment of 2-AG signaling that regulates synaptic activity and plasticity in key stressresponsive circuits, such as the BLA and nucleus accumbens. A failure to elevate eCB signaling, or in the case of AEA, a progressive loss of eCB signaling, in response to repeated stress exposure may limit appropriate buffer mechanisms in the brain to constrain the effects of stress and result in the development of stress susceptibility (see Fig. 22.1 for model). In line with this, there is also evidence that chronic stress compromises CB1 receptor expression and function (Hill et al., 2005; Wang et al., 2010; Zhong et al., 2014; Wamsteeker et al., 2010; Wamsteeker Cusulin et al., 2014), suggesting that a failure of CB1 receptors to maintain signaling capacity in the face of stress could also be a mechanism relating to susceptibility. Additional work is required to understand the role of CB1 receptor regulation in relation to outcomes from stress relating to pathology or resilience. While still in early days, there is some support from the human literature for this model. Specifically, chronic isolation stress exposure associated with reduced 2-AG levels in the circulation was found to correlate with reduced positive mood and elevated catecholamine levels (Yi et al., 2016). More so, stress-related psychiatric illnesses, such as major depression and PTSD, have been found to be associated with reduced eCB levels (Hill et al., 2008c, 2009b; 2013a; Neumeister et al., 2013). Based on these data, we predict that pharmacological approaches that augment eCB signaling may represent a novel approach to sculpt stress resilience. With the development, and initial human validation, of both FAAH (D’Souza et al., 2019) and MAGL (Cisar et al., 2018) inhibitors, we are hopeful that the potential utility of these compounds will be explored in the near future and their putative application in the context of stress-related psychiatry illnesses will be examined.

FIGURE 22.1 Repeated exposure to stress results in a progressive recruitment of endocannabinoid signaling (particularly 2-arachidonoylglycerol signaling), which in turn promotes synaptic plasticity and modifies synaptic transmission in key brain regions, such as the amygdala and nucleus accumbens, to promote stress resilience. A failure to elevate endocannabinoid (eCB) signaling, or a progressive loss of eCB signaling (as has been seen with anandamide), in response to repeated stress exposure may limit appropriate buffer mechanisms in the brain to constrain the effects of stress and result in the development of susceptibility to stress-related psychiatric disorders.

358

22. Endocannabinoid signaling and stress resilience

References Ahn, K., McKinney, M.K., Cravatt, B.F., 2008. Enzymatic pathways that regulate endocannabinoid signaling in the nervous system. Chemical Reviews 108 (5), 1687e1707. Aliczki, M., Balogh, Z., Tulogdi, A., Haller, J., 2012. The temporal dynamics of the effects of monoacylglycerol lipase blockade on locomotion, anxiety, and body temperature. Behavioural Pharmacology 23 (4), 348e357. Aliczki, M., Zelena, D., Mikics, E., Varga, Z.K., Pinter, O., Bakos, N.V., et al., 2013. Monoacylglycerol lipase inhibitioninduced changes in plasma corticosterone levels, anxiety and locomotor activity in male CD1 mice. Hormones and Behavior 63 (5), 752e758. Atsak, P., Hauer, D., Campolongo, P., Schelling, G., McGaugh, J.L., Roozendaal, B., 2012. Glucocorticoids interact with the hippocampal endocannabinoid system in impairing retrieval of contextual fear memory. Proceedings of the National Academy of Sciences of the United States of America 109 (9), 3504e3509. Atwood, B.K., Mackie, K., 2010. CB2: a cannabinoid receptor with an identity crisis. British Journal of Pharmacology 160 (3), 467e479. Bedse, G., Colangeli, R., Lavecchia, A.M., Romano, A., Altieri, F., Cifani, C., et al., 2014. Role of the basolateral amygdala in mediating the effects of the fatty acid amide hydrolase inhibitor URB597 on HPA axis response to stress. European Neuropsychopharmacology 24 (9), 1511e1523. Bedse, G., Hartley, N.D., Neale, E., Gaulden, A.D., Patrick, T.A., Kingsley, P.J., et al., 2017. Functional redundancy between canonical endocannabinoid signaling systems in the modulation of anxiety. Biological Psychiatry 82 (7), 488e499. Bedse, G., Bluett, R.J., Patrick, T.A., Romness, N.K., Gaulden, A.D., Kingsley, P.J., et al., 2018. Therapeutic endocannabinoid augmentation for mood and anxiety disorders: comparative profiling of FAAH, MAGL and dual inhibitors. Translational Psychiatry 8 (1), 92. Blankman, J.L., Simon, G.M., Cravatt, B.F., 2007. A comprehensive profile of brain enzymes that hydrolyze the endocannabinoid 2-arachidonoylglycerol. Chemical Biology 14 (12), 1347e1356. Bluett, R.J., Gamble-George, J.C., Hermanson, D.J., Hartley, N.D., Marnett, L.J., Patel, S., 2014. Central anandamide deficiency predicts stress-induced anxiety: behavioral reversal through endocannabinoid augmentation. Translational Psychiatry 4, e408. Bluett, R.J., Baldi, R., Haymer, A., Gaulden, A.D., Hartley, N.D., Parrish, W.P., et al., 2017. Endocannabinoid signaling modulates susceptibility to traumatic stress exposure. Nature Communications 8, 14782. Bosch-Bouju, C., Larrieu, T., Linders, L., Manzoni, O.J., Laye, S., 2016. Endocannabinoid mediated plasticity in nucleus accumbens controls vulnerability to anxiety after social defeat stress. Cell Reports 16 (5), 1237e1242. Bowles, N.P., Hill, M.N., Bhagat, S.M., Karatsoreos, I.N., Hillard, C.J., McEwen, B.S., 2012. Chronic, noninvasive glucocorticoid administration suppresses limbic endocannabinoid signaling in mice. Neuroscience 204, 83e89. Busquets-Garcia, A., Puighermanal, E., Pastor, A., de la Torre, R., Maldonado, R., Ozaita, A., 2011. Differential role of anandamide and 2-arachidonoylglycerol in memory and anxiety-like responses. Biological Psychiatry 70 (5), 479e486. Campos, A.C., Ferreira, F.R., Guimaraes, F.S., Lemos, J.I., 2010. Facilitation of endocannabinoid effects in the ventral hippocampus modulates anxiety-like behaviors depending on previous stress experience. Neuroscience 167 (2), 238e246. Carnevali, L., Vacondio, F., Rossi, S., Macchi, E., Spadoni, G., Bedini, A., et al., 2015. Cardioprotective effects of fatty acid amide hydrolase inhibitor URB694 in a rodent model of trait anxiety. Scientific Reports 5, 18218. Cisar, J.S., Weber, O.D., Clapper, J.R., Blankman, J.L., Henry, C.L., Simon, G.M., et al., 2018. Identification of ABX-1431, a selective inhibitor of monoacylglycerol lipase and clinical condition for treatment of neurological disorders. Journal of Medicinal Chemistry 61 (20), 9062e9084. Cravatt, B.F., Demarest, K., Patricelli, M.P., Bracey, M.H., Giang, D.K., Martin, B.R., et al., 2001. Supersensitivity to anandamide and enhanced endogenous cannabinoid signaling in mice lacking fatty acid amide hydrolase. Proceedings of the National Academy of Sciences of the United States of America 98 (16), 9371e9376. D’Souza, D.C., Cortes-Briones, J., Creatura, G., Bluez, G., Thurnauer, H., Deaso, E., et al., 2019. Efficacy and safety of a fatty acid amide hydrolase inhibitor (PF-04457845) in the treatment of cannabis withdrawal and dependence in men: a double blind, placebo-controlled, parallel group, phase 2a single-site randomised controlled trial. Lancet Psychiatry 6 (1), 35e45. Dallman, M.F., 2005. Adrenocortical function, feedback, and alphabet soup. American Journal of Physiology. Endocrinology and Metabolism 289 (3), E361eE362.

References

359

Danandeh, A., Vozella, V., Lim, J., Oveisi, F., Ramirez, G.L., Mears, D., et al., 2018. Effects of fatty acid amide hydrolase inhibitor URB597 in a rat model of trauma-induced long-term anxiety. Psychopharmacology 235 (11), 3211e3221. Devane, W.A., Hanus, L., Breuer, A., Pertwee, R.G., Stevenson, L.A., Griffin, G., et al., 1992. Isolation and structure of a brain constituent that binds to the cannabinoid receptor. Science 258 (5090), 1946e1949. Di, S., Malcher-Lopes, R., Marcheselli, V.L., Bazan, N.G., Tasker, J.G., 2005. Rapid glucocorticoid-mediated endocannabinoid release and opposing regulation of glutamate and gamma-aminobutyric acid inputs to hypothalamic magnocellular neurons. Endocrinology 146 (10), 4292e4301. Dincheva, I., Drysdale, A.T., Hartley, C.A., Johnson, D.C., Jing, D., King, E.C., et al., 2015. FAAH genetic variation enhances fronto-amygdala function in mouse and human. Nature Communications 6, 6395. Dinh, T.P., Carpenter, D., Leslie, F.M., Freund, T.F., Katona, I., Sensi, S.L., et al., 2002. Brain monoglyceride lipase participating in endocannabinoid inactivation. Proceedings of the National Academy of Sciences of the United States of America 99 (16), 10819e10824. Dubreucq, S., Matias, I., Cardinal, P., Haring, M., Lutz, B., Marsicano, G., et al., 2012. Genetic dissection of the role of cannabinoid type-1 receptors in the emotional consequences of repeated social stress in mice. Neuropsychopharmacology 37 (8), 1885e1900. Evanson, N.K., Tasker, J.G., Hill, M.N., Hillard, C.J., Herman, J.P., 2010. Fast feedback inhibition of the HPA axis by glucocorticoids is mediated by endocannabinoid signaling. Endocrinology 151 (10), 4811e4819. Fidelman, S., Mizrachi Zer-Aviv, T., Lange, R., Hillard, C.J., Akirav, I., 2018. Chronic treatment with URB597 ameliorates post-stress symptoms in a rat model of PTSD. European Neuropsychopharmacology 28 (5), 63e642. Fride, E., Suris, R., Weidenfeld, J., Mechoulam, R., 2005. Differential response to acute and repeated stress in cannabinoid CB1 receptor knockout newborn and adult mice. Behavioural Pharmacology 16 (5e6), 431e440. Gray, J.M., Vecchiarelli, H.A., Morena, M., Lee, T.T., Hermanson, D.J., Kim, A.B., et al., 2015. Corticotropin-releasing hormone drives anandamide hydrolysis in the amygdala to promote anxiety. Journal of Neuroscience 35 (9), 3879e3892. Gray, J.M., Wilson, C.D., Lee, T.T., Pittman, Q.J., Deussing, J.M., Hillard, C.J., et al., 2016. Sustained glucocorticoid exposure recruits cortico-limbic CRH signaling to modulate endocannabinoid function. Psychoneuroendocrinology 66, 151e158. Griebel, G., Stemmelin, J., Lopez-Grancha, M., Fauchey, V., Slowinski, F., Pichat, P., et al., 2018. The selective reversible FAAH inhibitor SSR411298, restores the development of maladaptive behaviors to acute and chronic stress in rodents. Scientific Reports 8 (1), 2416. Grissom, N., Bhatnagar, S., 2009. Habituation to repeated stress: get used to it. Neurobiology of Learning and Memory 92 (2), 215e224. Gulyas, A.I., Cravatt, B.F., Bracey, M.H., Dinh, T.P., Piomelli, D., Boscia, F., et al., 2004. Segregation of two endocannabinoid-hydrolyzing enzymes into pre- and postsynaptic compartments in the rat hippocampus, cerebellum and amygdala. European Journal of Neuroscience 20 (2), 441e458. Gunduz-Cinar, O., Hill, M.N., McEwen, B.S., Holmes, A., 2013. Amygdala FAAH and anandamide: mediating protection and recovery from stress. Trends in Pharmacological Sciences 34 (11), 637e644. Haller, J., Barna, I., Barsvari, B., Gyimesi Pelczer, K., Yasar, S., Panlilio, L.V., et al., 2009. Interactions between environmental aversiveness and the anxiolytic effects of enhanced cannabinoid signaling by FAAH inhibition in rats. Psychopharmacology (Berl) 204 (4), 607e616. Herkenham, M., Lynn, A.B., Johnson, M.R., Melvin, L.S., de Costa, B.R., Rice, K.C., 1991. Characterization and localization of cannabinoid receptors in rat brain: a quantitative in vitro autoradiographic study. Journal of Neuroscience 11 (2), 563e583. Hermanson, D.J., Hartley, N.D., Gamble-George, J., Brown, N., Shonesy, B.C., Kingsley, P.J., et al., 2013. Substrateselective COX-2 inhibition decreases anxiety via endocannabinoid activation. Nature Neuroscience 16 (9), 1291e1298. Hill, M.N., Bierer, L.M., Makotkine, I., Golier, J.A., Galea, S., McEwen, B.S., et al., 2013a. Reductions in circulating endocannabinoid levels in individuals with post-traumatic stress disorder following exposure to the world trade center attacks. Psychoneuroendocrinology 38 (12), 2952e2961. Hill, M.N., Carrier, E.J., Ho, W.S., Shi, L., Patel, S., Gorzalka, B.B., et al., 2008a. Prolonged glucocorticoid treatment decreases cannabinoid CB1 receptor density in the hippocampus. Hippocampus 18 (2), 221e226. Hill, M.N., Carrier, E.J., McLaughlin, R.J., Morrish, A.C., Meier, S.E., Hillard, C.J., et al., 2008b. Regional alterations in the endocannabinoid system in an animal model of depression: effects of concurrent antidepressant treatment. Journal of Neurochemistry 106 (6), 2322e2336.

360

22. Endocannabinoid signaling and stress resilience

Hill, M.N., Hillard, C.J., McEwen, B.S., 2011a. Alterations in corticolimbic dendritic morphology and emotional behavior in cannabinoid CB1 receptor-deficient mice parallel the effects of chronic stress. Cerebral Cortex 21 (9), 2056e2064. Hill, M.N., Karatsoreos, I.N., Hillard, C.J., McEwen, B.S., 2010a. Rapid elevations in limbic endocannabinoid content by glucocorticoid hormones in vivo. Psychoneuroendocrinology 35 (9), 1333e1338. Hill, M.N., Kumar, S.A., Filipski, S.B., Iverson, M., Stuhr, K.L., Keith, J.M., et al., 2013b. Disruption of fatty acid amide hydrolase activity prevents the effects of chronic stress on anxiety and amygdalar microstructure. Molecular Psychiatry 18 (10), 1125e1135. Hill, M.N., McLaughlin, R.J., Bingham, B., Shrestha, L., Lee, T.T., Gray, J.M., et al., 2010b. Endogenous cannabinoid signaling is essential for stress adaptation. Proceedings of the National Academy of Sciences of the United States of America 107 (20), 9406e9411. Hill, M.N., McLaughlin, R.J., Morrish, A.C., Viau, V., Floresco, S.B., Hillard, C.J., et al., 2009a. Suppression of amygdalar endocannabinoid signaling by stress contributes to activation of the hypothalamic-pituitary-adrenal axis. Neuropsychopharmacology 34 (13), 2733e2745. Hill, M.N., McLaughlin, R.J., Pan, B., Fitzgerald, M.L., Roberts, C.J., Lee, T.T., et al., 2011b. Recruitment of prefrontal cortical endocannabinoid signaling by glucocorticoids contributes to termination of the stress response. Journal of Neuroscience 31 (29), 10506e10515. Hill, M.N., Miller, G.E., Carrier, E.J., Gorzalka, B.B., Hillard, C.J., 2009b. Circulating endocannabinoids and N-acyl ethanolamines are differentially regulated in major depression and following exposure to social stress. Psychoneuroendocrinology 34 (8), 1257e1262. Hill, M.N., Miller, G.E., Ho, W.S., Gorzalka, B.B., Hillard, C.J., 2008c. Serum endocannabinoid content is altered in females with depressive disorders: a preliminary report. Pharmacopsychiatry 41 (2), 48e53. Hill, M.N., Patel, S., Carrier, E.J., Rademacher, D.J., Ormerod, B.K., Hillard, C.J., et al., 2005. Downregulation of endocannabinoid signaling in the hippocampus following chronic unpredictable stress. Neuropsychopharmacology 30 (3), 508e515. Hill, M.N., Tasker, J.G., 2012. Endocannabinoid signaling, glucocorticoid-mediated negative feedback, and regulation of the hypothalamic-pituitary-adrenal axis. Neuroscience 204, 5e16. Hillard, C.J., Edgemond, W.S., Jarrahian, A., Campbell, W.B., 1997. Accumulation of N-arachidonoylethanolamine (anandamide) into cerebellar granule cells occurs via facilitated diffusion. Journal of Neurochemistry 69 (2), 631e638. Howlett, A.C., Barth, F., Bonner, T.I., Cabral, G., Casellas, P., Devane, W.A., et al., 2002. International union of pharmacology. XXVII. Classification of cannabinoid receptors. Pharmacological Reviews 54 (2), 161e202. Karatsoreos, I.N., McEwen, B.S., 2011. Psychobiological allostasis: resistance, resilience and vulnerability. Trends in Cognitive Sciences 15 (12), 576e584. Kathuria, S., Gaetani, S., Fegley, D., Valino, F., Duranti, A., Tontini, A., et al., 2003. Modulation of anxiety through blockade of anandamide hydrolysis. Nature Medicine 9 (1), 76e81. Katona, I., Freund, T.F., 2012. Multiple functions of endocannabinoid signaling in the brain. Annual Review of Neuroscience 35, 529e558. Lomazzo, E., Bindila, L., Remmers, F., Lerner, R., Schwitter, C., Hoheisel, U., et al., 2015. Therapeutic potential of inhibitors of endocannabinoid degradation for the treatment of stress-related hyperalgesia in an animal model of chronic pain. Neuropsychopharmacology 40 (2), 488e501. Long, J.Z., Nomura, D.K., Cravatt, B.F., 2009. Characterization of monoacylglycerol lipase inhibition reveals differences in central and peripheral endocannabinoid metabolism. Chemical Biology 16 (7), 744e753. Marco, E.M., Viveros, M.P., 2009. The critical role of the endocannabinoid system in emotional homeostasis: avoiding excess and deficiencies. Mini Reviews in Medicinal Chemistry 9 (12), 1407e1415. Marrs, W.R., Blankman, J.L., Horne, E.A., Thomazeau, A., Lin, Y.H., Coy, J., et al., 2010. The serine hydrolase ABHD6 controls the accumulation and efficacy of 2-AG at cannabinoid receptors. Nature Neuroscience 13 (8), 951e957. Martin, M., Ledent, C., Parmentier, M., Maldonado, R., Valverde, O., 2002. Involvement of CB1 cannabinoid receptors in emotional behaviour. Psychopharmacology (Berl) 159 (4), 379e387. Matsuda, L.A., Lolait, S.J., Brownstein, M.J., Young, A.C., Bonner, T.I., 1990. Structure of a cannabinoid receptor and functional expression of the cloned cDNA. Nature 346 (6284), 561e564. McEwen, B.S., Gianaros, P.J., 2011. Stress- and allostasis-induced brain plasticity. Annual Review of Medicine 62, 431e445.

References

361

McLaughlin, R.J., Hill, M.N., Bambico, F.R., Stuhr, K.L., Gobbi, G., Hillard, C.J., et al., 2012. Prefrontal cortical anandamide signaling coordinates coping responses to stress through a serotonergic pathway. European Neuropsychopharmacology 22 (9), 664e671. McPartland, J.M., Glass, M., Pertwee, R.G., 2007. Meta-analysis of cannabinoid ligand binding affinity and receptor distribution: interspecies differences. British Journal of Pharmacology 152 (5), 583e593. Mechoulam, R., Parker, L.A., 2013. The endocannabinoid system and the brain. Annual Review of Psychology 64, 21e47. Metna-Laurent, M., Marsicano, G., 2015. Rising stars: modulation of brain functions by astroglial type 1 cannabinoid receptors. Glia 63 (3), 353e364. Morena, M., Patel, S., Bains, J.S., Hill, M.N., 2016. Neurobiological interactions between stress and the endocannabinoid system. Neuropsychopharmacology 41 (1), 80e102. Morgan, A.J., Kingsley, P.J., Mitchener, M.M., Altemus, M., Patrick, T.A., Gaulden, A.D., et al., 2018. Detection of cyclo-oxygenase-2 derived oxygenation products of the endogenous cannabinoid 2-arachidonoylglycerol in mouse brain. ACS Chemical Neuroscience 9 (7), 1552e1559. Naidu, P.S., Varvel, S.A., Ahn, K., Cravatt, B.F., Martin, B.R., Lichtman, A.H., 2007. Evaluation of fatty acid amide hydrolase inhibition in murine models of emotionality. Psychopharmacology (Berl) 192 (1), 61e70. Natividad, L.A., Buczynski, M.W., Herman, M.A., Kirson, D., Oleata, C.S., Irimia, C., et al., 2017. Constitutive increases in amygdalar corticotrophin-releasing factor and fatty acid amide hydrolase drive an anxious phenotype. Biological Psychiatry 82 (7), 500e510. Navarette, M., Diez, A., Araque, A., 2014. Astrocytes in endocannabinoid signaling. Philosophical Transactions of the Royal Society of London B Biological Sciences 369 (1654), 20130599. Neumeister, A., Normandin, M.D., Pietrzak, R.H., Piomelli, D., Zheng, M.Q., Gujarro-Anton, A., Potenza, M.N., Bailey, C.R., Lin, S.F., Najafzadeh, S., Ropchan, J., Henry, S., Corsi-Travali, S., Carson, R.E., Huang, Y., 2013. Elevated brain cannabinoid CB1 receptor availability in post-traumatic stress disorder: a positron emission tomography study. Mol Psychiatry 18 (9), 1034e1040. Newsom, R.J., Osterlund, C., Masini, C.V., Day, H.E., Spencer, R.L., Campeau, S., 2012. Cannabinoid receptor type 1 antagonism significantly modulates basal and loud noise induced neural and hypothalamic-pituitary-adrenal axis responses in male Sprague-Dawley rats. Neuroscience 204, 64e73. Patel, S., Hillard, C.J., 2008. Adaptations in endocannabinoid signaling in response to repeated homotypic stress: a novel mechanism for stress habituation. European Journal of Neuroscience 27 (11), 2821e2829. Patel, S., Kingsley, P.J., Mackie, K., Marnett, L.J., Winder, D.G., 2009. Repeated homotypic stress elevates 2-arachidonoylglycerol levels and enhances short-term endocannabinoid signaling at inhibitory synapses in basolateral amygdala. Neuropsychopharmacology 34 (13), 2699e2709. Patel, S., Roelke, C.T., Rademacher, D.J., Cullinan, W.E., Hillard, C.J., 2004. Endocannabinoid signaling negatively modulates stress-induced activation of the hypothalamic-pituitary-adrenal axis. Endocrinology 145 (12), 5431e5438. Patel, S., Roelke, C.T., Rademacher, D.J., Hillard, C.J., 2005. Inhibition of restraint stress-induced neural and behavioural activation by endogenous cannabinoid signalling. European Journal of Neuroscience 21 (4), 1057e1069. Pertwee, R.G., 2005. Pharmacological actions of cannabinoids. Handbook of Experimental Pharmacology 168, 1e51. Qin, Z., Zhou, X., Pandey, N.R., Vecchiarelli, H.A., Stewart, C.A., Zhang, X., et al., 2015. Chronic stress induces anxiety via an amygdalar intracellular cascade that impairs endocannabinoid signaling. Neuron 85 (6), 1319e1331. Rademacher, D.J., Meier, S.E., Shi, L., Ho, W.S., Jarrahian, A., Hillard, C.J., 2008. Effects of acute and repeated restraint stress on endocannabinoid content in the amygdala, ventral striatum, and medial prefrontal cortex in mice. Neuropharmacology 54 (1), 108e116. Ramikie, T.S., Nyilas, R., Bluett, R.J., Gamble-George, J.C., Hartley, N.D., Mackie, K., et al., 2014. Multiple mechanistically distinct modes of endocannabinoid mobilization at central amygdala glutamatergic synapses. Neuron 81 (5), 1111e1125. Ramikie, T.S., Patel, S., 2012. Endocannabinoid signaling in the amygdala: anatomy, synaptic signaling, behavior, and adaptations to stress. Neuroscience 204, 38e52. Roberts, C.J., Stuhr, K.L., Hutz, M.J., Raff, H., Hillard, C.J., 2014. Endocannabinoid signaling in hypothalamicpituitary-adrenocortical axis recovery following stress: effects of indirect agonists and comparison of male and female mice. Pharmacology Biochemistry and Behavior 117, 17e24.

362

22. Endocannabinoid signaling and stress resilience

Rossi, S., De Chiara, V., Musella, A., Sacchetti, L., Cantarella, C., Castelli, M., et al., 2010. Preservation of striatal cannabinoid CB1 receptor function correlates with the antianxiety effects of fatty acid amide hydrolase inhibition. Molecular Pharmacology 78 (2), 260e268. Rubino, T., Realini, N., Castiglioni, C., Guidali, C., Vigano, D., Marras, E., et al., 2008. Role in anxiety behavior of the endocannabinoid system in the prefrontal cortex. Cerebral Cortex 18 (6), 1292e1301. Russo, S.J., Murrough, J.W., Han, M.H., Charney, D.S., Nestler, E.J., 2012. Neurobiology of resilience. Nature Neuroscience 15 (11), 1475e1484. Sciolino, N.R., Zhou, W., Hohmann, A.G., 2011. Enhancement of endocannabinoid signaling with JZL184, an inhibitor of the 2-arachidonoylglycerol hydrolyzing enzyme monoacylglycerol lipase, produces anxiolytic effects under conditions of high environmental aversiveness in rats. Pharmacological Research 64 (3), 226e234. Sugiura, T., Kondo, S., Sukagawa, A., Nakane, S., Shinoda, A., Itoh, K., et al., 1995. 2-Arachidonoylglycerol: a possible endogenous cannabinoid receptor ligand in brain. Biochemical and Biophysical Research Communications 215 (1), 89e97. Sumislawski, J.J., Ramikie, T.S., Patel, S., 2011. Reversible gating of endocannabinoid plasticity in the amygdala by chronic stress: a potential role for monoacylglycerol lipase inhibition in the prevention of stress-induced behavioral adaptation. Neuropsychopharmacology 36 (13), 2750e2761. Surkin, P.N., Gallino, S.L., Luce, V., Correa, F., Fernandez-Solari, J., De Laurentiis, A., 2018. Pharmacological augmentation of endocannabinoid signaling reduces the neuroendocrine response to stress. Psychoneuroendocrinology 87, 131e140. Van Sickle, M.D., Duncan, M., Kingsley, P.J., Mouihate, A., Urbani, P., Mackie, K., et al., 2005. Identification and functional characterization of brainstem cannabinoid CB2 receptors. Science 310 (5746), 329e332. Wamsteeker, J.I., Kuzmiski, J.B., Bains, J.S., 2010. Repeated stress impairs endocannabinoid signaling in the paraventricular nucleus of the hypothalamus. Journal of Neuroscience 30 (33), 11188e11196. Wamsteeker Cusulin, J.I., Senst, L., Teskey, G.C., Bains, J.S., 2014. Experience salience gates endocannabinoid signaling at hypothalamic synapses. Journal of Neuroscience 34 (18), 6177e6181. Wang, M., Hill, M.N., Zhang, L., Gorzalka, B.B., Hillard, C.J., Alger, B.E., 2012. Acute restraint stress enhances hippocampal endocannabinoid function via glucocorticoid receptor activation. Journal of Psychopharmacology 26 (1), 56e70. Wang, W., Sun, D., Pan, B., Roberts, C.J., Sun, X., Hillard, C.J., et al., 2010. Deficiency in endocannabinoid signaling in the nucleus accumbens induced by chronic unpredictable stress. Neuropsychopharmacology 35 (11), 2249e2261. Yi, B., Nichoporuk, I., Nicolas, M., Schneider, S., Feuerecker, M., Vassilieva, G., et al., 2016. Reductions in circulating endocannabinoid 2-arachidonoyly glycerol levels in healthy humans subjects exposed to chronic stressors. Progress In Neuro-Psychopharmacology and Biological Psychiatry 67, 92e97. Zhong, P., Wang, W., Pan, B., Liu, X., Zhang, Z., Long, J.Z., et al., 2014. Monoacylglycerol lipase inhibition blocks chronic stress-induced depressive-like behaviors via activation of mTOR signaling. Neuropsychopharmacology 39 (7), 1763e1776.

Index Note: ‘Page numbers followed by “f” indicate figures, “t” indicates tables and “b” indicate boxes’. A

B

Acoustic startle response (ASR), 314 Actin polymerization, 7 Activational and organizational hypothesis, 87 Active coping skills, 25e26 Active resilience g-aminobutyric acid (GABA), 96 GAD65 haplodeficiency, 99e100 ventral hippocampus, 100e101 glutamic acid decarboxylase fear, 96e97 prepulse inhibition (PPI), 99 resilience, 97e99 stress, 96e97 two isozymes, 96 unconditioned stimulus (US), 98e99 posttraumatic stress disorder (PTSD), 95e96 Adaptive calibration model (ACM), 154e155 Adaptive chronic stress, 292 Adenosine triphosphate (ATP), 120 Adolescence, 201e202 Adolescent and adult stress models, 279 Adrenocorticotropic hormone (ACTH), 39f, 71 Adult hippocampal neurogenesis, 72e74 Adulthood, 202e203 Adult phenotypes, 150e154 Aging, 111 Alpha-beta hydrolase domain (ABHD), 350 AMPA receptor trafficking, 37 Amygdala, 327e329 BLA, 8, 47, 97, 221, 236, 328, 352 BNST, 236, 316 CeA, 236, 297, 298 Anterior cingulate cortex (ACC), 140e142 Aspire for flexibility, 27e28 Attention deficit hyperactivity disorder (ADHD), 81 Augmented/predictable maternal care, 168 Autism spectrum disorder (ASD), 81

Basolateral nucleus of the amygdala basolateral amygdala (BLA), 8, 47 basolateral nucleus, 236 Basolateral nucleus (BLA), 236 BDNF. See Brain-derived neurotrophic factor (BDNF) Bed nucleus of the stria terminalis (BNST), 236 Behavioral stress response, 234e237, 235f Biomarkers central nervous system (CNS), 311e312 experimental strategies, 312e315 potential additional biomarkers, 315e316 prospective strategies, 313e314 retrospective strategies, 314e315 Brain-derived neurotrophic factor (BDNF), 74, 96e97, 157 BDNF-mediated signaling, 8 Brain tissues, 37e38

C Calcium calmodulinedependent protein kinase II (CaMKII) dependent, 238 Castrated males, 87 Catechol-O-methyltransferase (COMT), 137 Central nervous system (CNS), 311e312 Central nucleus of the amygdala (CeA), 236, 297, 298, 303 Child abuse/neglect future, 190e191 hypothalamic-pituitary-adrenal axis childhood maltreatment influence, 184e185 physiology, 184 sympathetic nervous system responses, 184e185 implications, 190e191 stress responsivity CRFR1/OPRL1/5HTLPR/BDNF/NPY/DHEA, 189e190 epigenetics, 186e187

363

364 Child abuse/neglect (Continued ) glucocorticoid feedback regulation, 185e186 inflammation, 188 neural circuits, 187e188 physiology, 182e185 resilience, 189 sympathetic nervous system, 185 treatment, 190e191 Childhood, 200e201 Chromatin looping, 270b Chromatin modifications, 216e217 Chronic inflammatory stress, 112e113 Chronic social defeat stress (CSDS), 209e211 Chronic stress, 8e9, 85e86, 237e239, 292e293 Chronic variable stress (CVS), 209e211 Circadian rhythm, 108e109 Circuit functions, 170e174 Circuit-related molecules, 221e222 Cognitive and behavioral components active coping skills, 25e26 cognitive flexibility, 25 optimism, 24e25 personal moral compass, 26e27, 29e30 physical activity, 26 psychosocial factors aspire for flexibility, 27e28 face your fears, 28 optimism, 27e28 pessimism, 27e28 physical health, 29 resilient role model, 28 supportive social network, 29 well-being, 29 social support network, 25e26 Cognitive and emotional outcomes, 168e170 Cognitive consequences, 168e169 Cognitive flexibility, 25 Cognitive function, rodents, 47e49 Conditioned response (CR), 323e324 Conditioned stimuli (CS), 36, 323e324 Connor-Davidson Resilience Scale (CDRISC), 134e135, 134f Context of susceptibility, 354e356 Corticosterone levels, 8 Corticotropin-releasing factor (CRF), 9, 136e137, 153, 244, 351 Cortisol rhythms, 113e114 CRF. See Corticotropin-releasing factor (CRF) CRFR1 availability, stress regulation, 242e243 CRF receptor 1 gene (CRHR1), 137e138 CRFR2 expression, 243e244 CRF-urocortin system basolateral nucleus (BLA), 236

INDEX

bed nucleus of the stria terminalis (BNST), 236 behavioral stress response, 234e237, 235f calcium calmodulinedependent protein kinase II (CaMKII) dependent, 238 central nucleus (CeA), 236 chronic stress exposure, 237e239 corticotropin-releasing proteinebinding protein function, 244 CRFR1 availability, stress regulation, 242e243 CRFR2 expression, 243e244 epigenetic regulation, 240e242 genetic variance x environment interactions, 239e240 glucocorticoid receptor (GR), 237e238 intracellularly activated signaling pathways alterations, 244e245 microRNAs (miRs), 241e242 overexpression (OE), 233e234 paraventricular nucleus (PVN), 233e234, 236

D Depression. See Major depressive disorder (MDD) Developmental origins of health and disease (DOHaD), 258 Disrupted maternal care, 166e168 DNA methylation, 214e216, 214fe215f, 270b DNA sequence, 276 Dopaminergic projections, 39 Drugs of abuse, 281 Dutch Hunger Winter, 258e259

E Early-life experiences, 14e15, 172 augmented/predictable maternal care, 168 behavioral functions, 170e174 circuit functions, 170e174 cognitive and emotional outcomes, 168e170 cognitive consequences, 168e169 disrupted maternal care, 166e168 early-life experiences, 172 emotional consequences, 169e170 gene-environment interaction, 165 glucocorticoid receptor (GR), 172 lateral habenula (LHb), 172 memory consequences, 171 neuronal functions, 170e174 nucleus accumbens (NAc), 172 paraventricular nucleus (PVN), 170e171 stress-sensitive neurons, 170e171 Early-life stress (ELS), 278e279 adaptive calibration model (ACM), 154e155 adult phenotypes, 150e154 rationale for shaping, 154e155

INDEX

brain-derived neurotrophic factor (BDNF) expression, 157 corticotropin-releasing factor (CRF), 153 definition, 149e150 FK506 binding protein 51 (FKBP51), 151 glucocorticoid receptor (GR) activity, 151 glucocorticoids (GCs), 150e151 major depressive disorder (MDD), 150 posttraumatic stress disorder (PTSD), 150 psychiatric disorders, risk factor, 150 single nucleotide polymorphism (SNP), 151 Trier Social Stress Test (TSST), 155 ventral tegmental area (VTA), 153 ventromedial prefrontal cortex (vmPFC), 152 Early postnatal period, 275b Early prenatal stress (EPS), 89 Electron transfer chain (ETC), 120 Elevated plus maze (EPM), 314 Emotional consequences, 169e170 Emotional experiences, 33 Emotional homeostasis, 352e353 Endocannabinoids, 9e10 signaling alpha-beta hydrolase domain (ABHD), 350 context of susceptibility, 354e356 cortictropin-releasing factor (CRF), 351 resilience, 354e356 stress, 351e352 stress response regulation, 352e354 regulation, 352e354 Environmental enrichment (EE), 263e264, 280 Epigenetics, 12e13 chromatin looping, 276b definition, 269 DNA methylation, 276b DNA sequence, 270 drugs of abuse, 275 early postnatal period, 282b germline-dependent transmission, 271 histone posttranslational modifications, 276b inherited effects of stress, 272e273 adolescent and adult stress models, 273 early life stress, 272e273 environmental enrichment, 273 in utero, 272 inter- and transgenerational stress effects, 273e274 postnatal stress, 274 in utero, 273e274 molecular basis, 276b noncoding RNAs, 276b nonegermline transmission, 271e272 other environmental factors, 274e275 prenatal period, 282b

365

regulation, 240e242 small noncoding RNAs (sncRNAs), 276b stress for society, 275e276 transfer RNA (tRNA), 276b Epigenetic regulation, 240e242 Estradiol, 87 Estrogen receptor alpha gene (ESR1), 201 Excitatory amino acids, 8 Eyeblink conditioning, 37

F Fatty acid amide hydrolase (FAAH), 9e10 Fears, 28 FK506 binding protein 51 (FKBP51), 151 Flavoadenine dinucleotide (FADH2), 120 Foundational populations, 258e259

G GABAB receptors adult hippocampal neurogenesis, 72e74 alterations in, 65e66 antidepressants, rodents clinical evidences, 65 density, 64 function, 64e65 brain-derived neurotrophic factor (BDNF), 74 depression-like behaviors, 66 GABAB1 receptor subunit isoforms, 67e69, 67f hypothalamic-pituitary-adrenal axis, 71 location, 71e72 mechanisms, 69e74 serotonin (5-HT) neurotransmitter system, 69e70 stress-related psychiatric disorders, 64e66 GABAB1 receptor subunit isoforms, 67e69, 67f GAD65/67, 96e97, 99e101 Gene-environment interaction, 165 Gene-environment wide interaction studies (GEWIS), 203 Gene expression, 11e12 Generalized anxiety disorder (GAD), 323 Genetic disposition, 99e100 Genetics, 136e139 Genetic variation, 49e53 Genome-wide association studies (GWAS), 197 Genome-wide expression profiling, 10e11 Genome-wide studies, 222 Genome-wide unbiased studies, 138e139 Germline-dependent transmission, 277 GluA1, 39 Glucocorticoids (GCs), 8, 10e11, 150e151 exposure, 85e86 feedback regulation, 185e186

366 Glucocorticoids (GCs) (Continued ) glucocorticoid receptor (GR), 46, 49, 151, 172, 237e238, 258e259 aging, 111 chronic inflammatory stress, 112e113 exposure to constant light, 113 gender, 111 genetic background, 112 neonatal programming, 113 pathological conditions, 110e113 physiological conditions, 110e113 reproductive cycle, 111e112 stress hormone responses, 36e37 Gonadal testosterone, 87 Good stress, 2

H Harsh language, 14 Heart rate variability (HRV), 140 High-intensity unconditioned stimulus (US), 98e99 Histone deacetylase (HDAC), 13 Histone posttranslational modifications, 270b Holocaust survivor offspring (HSO), 258e259 HPA axis. See Hypothalamic -pituitary -adrenal (HPA) axis 5-HT1A receptor, 70 11b-Hydroxysteroid dehydrogenase type 2 (HSD-2), 46 Hypothalamic -pituitary -adrenal (HPA) axis, 33e34, 45, 49e51, 71, 82, 137, 258e259, 352 amygdala projections, 299e303 BST, 301 childhood maltreatment influence, 184e185 chronic stress resilience, 292e293 glucocorticoid signaling, 291e293 hippocampal projections, 299e303 hypothalamic and brain stem circuitry, 302e303 hypothalamoepituitaryeadrenocortical (HPA) axis, 291e292 limbic regulation, 293e299 amygdala, 293e299 general organizational scheme, 293e295, 294f hippocampus, 293e299, 296t medial prefrontal cortex, 298e299 prefrontal cortex, 293e299 maladaptative chronic stress responses, 292 neurocircuitry, 303e305 paraventricular nucleus (PVN), 291e292 paraventricular thalamus (PVT), 301e302 physiology, 184 prefrontal projections, 299e303 sympathetic nervous system responses, 184e185 Hypothesized mechanisms of transmission, 261e262

INDEX

I Immune-related processes, 220 Infancy, 199e200 Inflammation, 188 Intergenerational transmission foundational populations, 258e259 hypothesized mechanisms of transmission, 261e262 maternal transmission, 259e260 paternal transmission, 260e261 resilience, 262e264 Intervention, 15 Intracellularly activated signaling pathways alterations, 244e245

L Lateral habenula (LHb), 172 Learning-to-cope training brain tissues, 37e38 conditioned stimulus (CS), 36 definition, 34e35 emotional experiences, 33 GluA1, 39 hormones and behavior, 35e37 hypothalamicpituitary-adrenal (HPA) axis, 33e34 limitations, 40 synaptic plasticity, 37 unconditioned stimulus (US), 36 O-linked N-acetylglucosamine transferase (OGT), 89

M Magnetic resonance imaging (MRI), 312 Magnetic resonance spectroscopy (MRS), 122e123 Major depressive disorder (MDD), 52, 84e85, 150 Maladaptative chronic stress, 292 Maladaptive learning amygdala, 327e329 amygdalaeprefrontal communication directionality, 333e334 overview, 332e333 recall of learned associations, 335e337 cognitive and physiological components, 325e327 conditioned response (CR), 323e324 conditioned stimuli (CS), 323e324 generalized anxiety disorder (GAD), 323 medial prefrontal cortex mixed selectivity encoding, 329e331, 330f prelimbic and infralimbic subregions, 331e332 mPFCeBLA circuit function, 337e338 unconditioned stimuli (US), 323e324 Maternal transmission, 259e260 Matrix metalloproteinase-9 (MMP-9), 7 MDD. See Major depressive disorder (MDD)

INDEX

Medial prefrontal cortex mixed selectivity encoding, 329e331, 330f prelimbic and infralimbic subregions, 331e332 Memory consequences, 171 Metabolomics and proteomics approaches, 124e125 MicroRNAs (miRs), 217, 241e242 Mineralocorticoid receptor (MR), 46, 49 cognitive function, rodents, 47e49 11b-hydroxysteroid dehydrogenase type 2 (HSD-2), 46 hypothalamus-pituitary-adrenal (HPA) axis activity, 45, 49e51 neuronal activity, 46e47 nucleus tractus solitarii (NTS), 46 psychopathology, humans genetic variation, 49e53 learning, 51e52 pharmacology, 49e53 psychiatric disorders, 52e53 resilience, 52e53 stress appraisal, 51e52 vulnerability, 49e53 Mitochondrial function adenosine triphosphate (ATP), 120 electron transfer chain (ETC), 120 flavoadenine dinucleotide (FADH2), 120 future perspectives, 127 glucocorticoids, 121e122 mitochondrial DNA (mtDNA), 120 neurotransmission, 121 posttraumatic stress disorder (PTSD), 119 reactive oxygen species (ROS), 120e121 singlenucleotide polymorphisms (SNPs), 123e124 Sirtuin 1 (SIRT1), 124 stress effects, animal studies, 124e126 stress-induced psychopathology, 119 stress-related disorders, 122e124 stress resilience, 126 synaptic plasticity, 121 Mitochondria-targeted antioxidant (MitoQ), 125e126 Molecular characterization, resilient brain chromatin modifications, 216e217 chronic social defeat stress (CSDS), 209e211 chronic variable stress (CVS), 209e211 circuit-related molecules, 221e222 DNA methylation, 214e216, 214fe215f future directions, 224e225 genome-wide studies, 222 immune-related processes, 220 microRNAs, 217 neurotrophic factors, 220e221 psychiatric disorders, 209 transcription factors, 218, 219f Multidimensional trait, 142, 142f

367

N Neonatal programming, 113 Neural circuits, 187e188, 293 Neuroactive steroids, 88 magnetic resonance imaging (MRI), 312 magnetic resonance spectroscopy (MRS), 122e123 positron emission tomography (PET), 122e123 Neuroimaging, 140e142 Neuroligins (NLGNs), 7 Neuronal activity, 46e47 Neuronal functions, 170e174 Neuropsychiatric disorders, 81 Nicotinamide adenine dinucleotide (NADH), 120 N-methyl-D-aspartate (NMDA), 138 Noncoding RNAs, 270b Nonegermline transmission, 277e278 Nucleus accumbens (NAc), 172 Nucleus tractus solitarii (NTS), 46 NUP-62, 7

O Optimism, 24e25, 27e28 Overexpression (OE), 233e234 Oxidative phosphorylation (OXPHOS) system, 120 OXTR polymorphisms, 199e200 Oxytocin peptide (OXT), 199e200 Oxytocin receptor gene (OXTR), 138

P Paraventricular nucleus (PVN), 170e171, 233e234, 236 Paternal transmission, 260e261 Peripartum depression, 85e86 Personal moral compass, 26e27, 29e30 Pessimism, 27e28 Physical activity, 26 Pituitary adenylate cyclase-activating polypeptide (PACAP), 136e137 Populations, biological approaches candidate studies, 136e138 Connor-Davidson Resilience Scale (CDRISC), 134e135, 134f genetics, 136e139 genome-wide unbiased studies, 138e139 multidimensional trait, 142, 142f neuroimaging, 140e142 physiology, 139e140 resilience, 134e136 serotonin (5HT) transporter (5HTT), 136 Positive allosteric modulators (PAMs), 66 Positron emission tomography (PET), 122e123 Posttraumatic stress disorder (PTSD), 23, 84e85, 150, 258 Potential additional biomarkers, 315e316

368 Predatorscent-stress (PSS) exposure, 10e11 Prenatal development, 198e199 Prolonged exposure (PE), 140 Psychiatric disorders, 52e53 adolescence, 201e202 adulthood, 202e203 childhood, 200e201 infancy, 199e200 prenatal development, 198e199 Psychiatric disorders, risk factor, 150 Psychiatric Genomics Consortium (PGC), 124 PTSD. See Posttraumatic stress disorder (PTSD) PVN. See Paraventricular nucleus (PVN)

R Reactive oxygen species (ROS), 120e121 Religion/spirituality, 26e27 Resilience, 189, 262e264 epigenetics, 1 stress, 1 Resilient role model, 28 Retrospective strategies, 314e315 Reversibility, 15 Rhythms Circadian rhythm, 108e109 cortisol rhythms, 113e114 glucocorticoid rhythms aging, 111 chronic inflammatory stress, 112e113 exposure to constant light, 113 gender, 111 genetic background, 112 neonatal programming, 113 pathological conditions, 110e113 physiological conditions, 110e113 reproductive cycle, 111e112 hormonal and behavioral response, 109e110, 110f hypothalamic-pituitary-adrenal axis rhythms, 107

S Schizophrenia, 81, 99 Selective serotonin reuptake inhibitor (SSRI), 69e70 Sex chromosomes, 88e89 Sex differences, 11 attention deficit hyperactivity disorder (ADHD), 81 autism spectrum disorder (ASD), 81 hypothalamic-pituitary-adrenal (HPA) axis, 82 neuropsychiatric disorders, 81 schizophrenia, 81 sex chromosome, 88e89 sex hormone, 87e88

INDEX

sex life span interaction, 84e86 stress pathway dysregulation, 82 Sex hormone, 87e88 Sex life span interaction, 84e86 Single nucleotide polymorphisms (SNPs), 123e124, 136e137, 151 Sirtuin 1 (SIRT1), 124 Skin conductance, 140 Small noncoding RNAs (sncRNAs), 270b Spontaneous hypertensive rats (SHRs), 55 Stress, 2, 99 Stress appraisal, 51e52 Stress habituation, 354e355 Stress mediators, 54 Stress pathway dysregulation, 82 Stress-related phenotypes, 52e53 Stress-related psychiatric disorders, 64e66 Stress responsivity CRFR1/OPRL1/5HTLPR/BDNF/NPY/DHEA, 189e190 epigenetics, 186e187 glucocorticoid feedback regulation, 185e186 inflammation, 188 neural circuits, 187e188 physiology, 182e185 resilience, 189 Stress-sensitive neurons, 170e171 Supportive social network, 29 Sympathetic nervous system, 185 Synaptic AMPA receptor trafficking, 39 Synaptic plasticity, 37

T Theta-range synchrony, 336 Tolerable stress, 2 Toxic stress, 2 Transcription factors, 218, 219f Transfer RNA (tRNA), 270b Trier Social Stress Test (TSST), 155

U Urocortin, 233, 234, 235, 237, 239 Unconditioned stimuli (US), 36, 323e324

V Ventral hippocampus (vHi), 73 Ventral tegmental area (VTA), 71e72, 153 Ventromedial prefrontal cortex (vmPFC), 152 Vulnerability, 49e53

W Well-being, 29